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INTERIOR WIRING 

AND SYSTEMS FOR 

ELECTRIC LIGHT AND POWER SERVICE 

A MANUAL OF PRACTICE 
FOR 

ELECTRICAL WORKERS, CONTRACTORS, 
ARCHITECTS AND SCHOOLS 


BY 

ARTHUR L. <pOOK 

Head of Department of Applied Electricity, Pratt Institute 
Formerly Electric Power Engineer, Westinghouse, Church, Kerr & Co. 


FIRST EDITION 


NEW YORK 

JOHN WILEY & SONS, Inc. 
London: CHAPMAN & HALL, Limited 

1917 


1 ffsa7/ 

(o 



Copyright, 1917 

BY 

ARTHUR L. COOK 





A 

MAR -8 1917 


PRESS OF 

BRAUNWORTH & CO. 

BOOK MANUFACTURERS 
BROOKLYN, N. Y. 

©a A 4 558 51 

( . 


PREFACE 


;is book is intended as a guide to modern practice in elec¬ 
tric lighting and power applications, and in the design and in¬ 
stallation of the wiring for such purposes. 

It has been written particularly for electrical workers who 
wish to become familiar with good practice in these branches 
of the electrical industry. The information presented is in¬ 
tended especially for superintendents and operators of electrical 
installations and for wiremen, who may be called upon to make 
extensions to or changes in existing installations, and who need 
definite information as to the best method of procedure. This 
book is also adapted for use in schools giving trade courses in 
electric wiring, to supplement the instruction given in the 
shop. It should also be useful for architects, when planning 
electrical installations. The tables in Appendix A and the 
methods of calculating wiring circuits will be of particular service 
to electrical contractors, when laying out new work. 

There are many text-books which deal with the principles 
of operation of electrical apparatus and the methods of cal¬ 
culating electric circuits, but the usual electrical worker or the 
student does not possess a sufficient background of prac¬ 
tical experience to enable him to use these principles to design 
a wiring installation. The attempt has been made here 
to take up these subjects where they are left by the text¬ 
books and to supply information which will compensate, in 
part at least, for a lack of practical experience. The inten¬ 
tion has been to make the information as complete as possible 
so that the user may have sufficient data to meet his needs. 
Accordingly, lighting and power applications are covered in 
Parts I and II in such a way that the reader can gain an idea 
of the types of lamps and motors available, and the proper 
applications of each type, Simple rules are given for deter- 

iii 



IV 


PREFACE 


mining the size and arrangement of lighting units and for 
determining the sizes of motors required for various kinds of 
service. Following this, in Part III, is a description of apparatus 
and fittings for interior wiring, together with detailed methods 
of calculating the sizes of circuits to meet various requirements. 
The examples given in the last chapter illustrate and describe 
typical lighting and power applications and are reproduced 
from actual installations. Throughout the book, examples are 
included to illustrate each step. The calculations are taken 
up in detail, from the switchboard to the lamps, and the method 
of planning each part of the circuit is explained. This is car¬ 
ried out completely for each system—two-wire, three-wire, 
three-phase and two-phase. 

This method requires the use of a large amount of detailed 
information and involves the use of definite rules of procedure. 
While, sometimes, these methods cannot be used without con¬ 
siderable modification, there is so much similarity in the usual 
industrial applications that generally their use will give satis¬ 
factory results. To assist in making an intelligent application 
and to guard against improper use of the data given, the limita¬ 
tions existing in each case are carefully stated. Explanations 
are also given of the performance of different lamps and motors 
under various conditions so that satisfactory applications 
may be secured 

No attempt has been made to explain, in detail, the theory 
of operation of the various motors, as it would be beyond the 
scope of this book. Because it is written for “ practical men,” 
only simple mathematics have been used in the calculations.' 
The methods employed for alternating-current circuits are 
therefore somewhat limited in their application. For the 
usual problems met with in practice, however, they are entirely 
satisfactory and care has been taken to point out such limita¬ 
tions as exist, so that the user may know whether or not these 
methods can be safely employed. 

Free use has been made of illustrations and diagrams, 
most of which have been drawn especially for this book. In 
describing apparatus, it has been necessary to use many illus¬ 
trations furnished by electrical manufacturers. It would 
obviously be impossible to show a complete line of apparatus 
because of the great variety on the market. The fact, there¬ 
fore, that only a single illustration, typical of many products 


PREFACE 


V 


available, has been given, should not lead the reader to con¬ 
clude that similar apparatus made by other manufacturers 
is not as good. The intention has been to illustrate each type 
of apparatus only, leaving the reader to obtain detailed infor¬ 
mation on a particular line of apparatus from trade catalogues. 
With a few exceptions, the tables in Appendix A were prepared 
especially for this book. They are intended to be “ working 
tables ” which can be used to save long computations and to 
give definite information on the various problems involved in 
the lighting and power applications. A few of these tables and 
the chart for the calculation of voltage drop were first published 
by the author in the magazine Power and are reproduced here 
by permission. 

Thanks are due the numerous manufacturing concerns that 
have assisted by furnishing illustrations or performance infor¬ 
mation, and particularly to the Westinghouse Electric and 
Manufacturing Co., the General Electric Co., the Cooper 
Hewitt Electric Co., and Westinghouse, Church, Kerr & Co. 
The author has tried to make the information presented as com¬ 
plete and accurate as possible. He will appreciate any suggested 
improvements or notices of errors in the data given. 

Arthur L. Cook. 

Brooklyn, New York, 

December, 1916. 



























. 










































■ 














• l >1 .'I 1 





















TABLE OF CONTENTS 


PART I. ELECTRIC LIGHTING SYSTEMS 

CHAPTER PAR. PAGE 

1. Introduction. 1- 5 1 

2. Incandescent Lamps. 6-35 4 

Carbon—gem—tungsten—construction and 
performance. 

3. Arc Lamps. 36- 62 17 

Open and enclosed arcs—flame-arcs—me¬ 
tallic-electrode arcs—mercury-vapor lamps. 

4. Principles of Illumination. 63- 77 37 

Light—color—reflection—units—distribution 

curves—requirements for illumination. 

5. Lighting Accessories. 78- 98 51 

Reflectors—globes—shades—construction and 
applications. 

6. Lighting Fixtures. 99-108 64 

Types—fixtures for direct, semi-indirect and 

indirect lighting—supports. 

7. Practical Methods of Calculating In¬ 

terior Illumination. 109-131 78 

Systems—type of lamps—intensity—uniform 
illumination—power required—size and loca¬ 
tion of unit—examples. 

8. Outdoor Lighting. 132-142 111 

Street lighting—yards—tennis courts—signs 

— flood lighting. 

PART II. ELECTRIC POWER SYSTEMS 

9. Motors for Industrial Purposes. 143-170 124 

Advantages of the electric drive—shunt, series 

and compound motors—applications—induc¬ 
tion motors—single-phase motors—synchro- 


VII 












Vlll 


TABLE OF CONTENTS 


CHAPTER 

nous motors—applications—comparison— 
standard voltages and frequencies—rating 
and overload capacity—motor performance. 

10. Motor-Starting Devices and Control¬ 

lers . 

Starting methods—rating—starters for d.c. 
and a.c. motors—dynamic braking. 

11. Selecting Motors for Industrial Pur¬ 

poses. 

Methods of driving machines—choosing type 
and speed—load and motor rating—require¬ 
ments of machines—belt drives—gear drives 
—chain drives. 


PAR. PAGE 


171-186 155 


187-211 172 


PART III. INTERIOR WIRING 


12. Systems of Wiring. 212-220 190 

D.c systems—a.c. systems—comparison and 

choice. 

13. Methods of Installing Wiring. 221-253 208 


Rigid conduit systems—flexible conduit sys¬ 
tems—armored cable systems—metal mould¬ 
ing—wood moulding—knob and tube system 
—open work—comparison of systems— 
wiring for special conditions. 

14. Wires and Cables.. 254-262 248 

Copper wire—wire gauges—stranded con¬ 
ductors—rubber and weatherproof insulation 

—multiple conductors—carrying capacity— 
splicing. 

15. Switches, Circuit Breakers and Fuses. 263-285 261 

Knife switches—snap switches—push-button 

switches—air-break circuit breakers—oil cir¬ 
cuit breakers—open fuses—enclosed fuses— 
cutouts. 


16. Sockets and Receptacles. 286-290 280 

Key and keyless sockets—weatherproof 

sockets—rating—rosettes—plug receptacles. 

17. Panel Boards and Switchboards. 291-297 285 


Lighting panel boards—power panel boards— 
switchboards. 









TABLE OF CONTENTS 


IX 


CHAPTER PAR. PAGE 

18. Arrangement of Circuits . 298-308 293 


Feeder systems—control of branch circuits— 
arrangement of branch circuits—location of 
panel boards and switchboards—arrange¬ 


ment of feeders—grounding. 

19. Calculation of D.C. Systems . 309-320 309 

Calculation of load—determining size of 

branch circuits, feeders and mains—fusing— 
calculation of voltage loss. 

20. Calculation of A.C. Systems . 321-336 321 


Self-induction—skin effect—power factor— 
grouping of conductors—calculation of load, 
three-phase and two-phase—combining loads 
having different power factors—determining 
size of branch circuits, feeders and mains 
—fusing—calculation of voltage loss. 

21. Examples of Wiring Systems . 337-351 337 

Office building-—collar factory—machine shop 
—railroad repair shop—examples of group 
and individual drives—hotel—residence. 


TABLES IN APPENDIX A 

TABLE PAGE 

1. Data on Metalized-filament Lamps. 353 

2. Data on Mazda B Tungsten Lamps. 353 

3. Data on Mazda C Tungsten Lamps. 354 

4. Performance of Enclosed Arc Lamps. 355 

5. Performance of Flame-arc Lamps. 356 

6. Performance of Metallic-electrode Arc Lamps. 357 

7. Performance of Cooper Hewitt Lamps.. :. 358' 

8. Illumination Intensities for Commercial Lighting. 359' 

9. Illumination Intensities for Industrial Lighting. 362 

10. Utilization Efficiencies for Tungsten Lamps. 367 

11. Color Classification of Walls and Ceilings. 368 

12. Power Required to Produce One Foot-candle Illumi¬ 

nation. 368 

13. Power Required for Tungsten Lighting Systems.369 

14. Power Required for Flame-arc Lighting Systems.. .... 370 

15. Sizes of Lighting Units for Various Mounting Heights.. 370 

16. Power Required for Lighting with Cooper Hewitt 

Lamps. 371 





















X 


TABLE OF CONTENTS 


TABLE PAGE 

17. Desirable Spacing for Direct Lighting Units. 372 

18. Desirable Spacing for Indirect and Semi-indirect Light¬ 

ing Units. 373 

19. Illumination Intensities for Street Lighting. 373 

20. Temperature Ratings and Overloads of Motors. 374 

21. Current and Size of Wire for D.C. Motors.. 375 

22. Current and Size of Wire for Three-phase Induction 

Motors. 376 

23. Current and Size of Wire for Two-phase Induction 

Motors.. 377 

24. Power Factor of Induction Motors. 378 

25. Usual Speed Ratings of Motors. 378 

26. Standard Pulley Sizes for Motors. 379 

27. * Horsepower Transmitted by Leather Belts. 380 

28. Dimensions of Iron Conduit and Elbows. 382 

29. Dimensions of Locknuts and Bushings. 383 

30. Sizes of Iron Conduit for Rubber-covered Wires. 384 

31. Sizes of Pull Boxes. 385 

32. Spacing of Anchors on Vertical Runs. 385 

33. Dimensions of B. and D. Cleats. 386 

34. Dimensions of Bare Stranded Cables. 387 

35. Dimensions of Insulated Wires. 388 

36. Current-carrying Capacity of Wires. 389 

37. Dimensions of Lighting Panel Board Cabinets. 390 

38. Demand Factors for Motor Loads. 391 

39. Sizes for Fuses for Motors. .. 391 

40. Values of Maximum Voltage Drop. 392 

41. Branch Lighting Circuits. 392 

42. Skin Effect for Round Copper Conductors. 393 

43. Power Factors of Apparatus. 394 

44. Reactive and Resistance Factors. 394 

45. Ratio of Reactance to Resistance. 395 

46. Drop Factors. 398 

47. Symbols for Wiring Diagrams. 400 

48. Standard Symbols for Wiring Plans,. 401 

Index. 403 



































INTERIOR WIRING 

AND SYSTEMS FOR 

ELECTRIC LIGHT AND POWER SERVICE 


PART I. ELECTRIC LIGHTING SYSTEMS 


CHAPTER 1 

INTRODUCTION 

/' 

1. Methods of Producing Artificial Light. In general, light 
is produced by heating a solid or gaseous body to a high tem¬ 
perature, although this is not always necessary, as is shown 
by the action of vacuum-tube and mercury-vapor lamps. 
When a body is heated, it gives off heat as well as light, and 
sufficient energy must be supplied to produce both of these 
effects. For a high light-producing efficiency, the heat energy 
(which is wasted) must be as small as possible. 

2. Efficiency of the Electric Light. As a substance is 
heated, the color of the light produced changes from red to 
white as the temperature increases, and the light efficiency 
increases very rapidly. High efficiency therefore necessitates 
operation at high temperatures.* It is difficult, however, to 
find substances which will withstand the required high tem¬ 
perature without melting or vaporizing. The high efficiency 
of the tungsten lamp results from the high temperature at which 
the filament may be operated without rapid deterioration. 
The high efficiencies obtainable from arc lamps are possible 
only because of the very high temperature of the electric arc. 
In spite of all that has been done to improve this efficiency, 
the amount of energy given off in the form of light from a “ gas- 
filled ” tungsten lamp is only about 3.5 per cent of the total 


* Except as noted in paragraph 1. 




2 


INTRODUCTION 


[chap. 1 


amount of energy supplied to the lamp. The remainder of 
the energy disappears in the form of heat. This lamp is the 
most efficient type of incandescent lamp and is exceeded in 
efficiency only by certain kinds of arc lamps which convert 
about 4.5 per cent of the energy into light.* 

3. Classes of Lamps. Incandescent lamps produce light 
by the heating effect of an electric current traversing a solid 
conductor, and always operate at a high temperature. In 
arc lamps, the current passes through a gaseous conductor, 
and the light proceeds either from the gas or from the solid 
terminals where the gaseous column makes connection with 
the circuit. The production of light by means of the electric 
arc is accompanied by a very high temperature, except in 
vacuum-tube lamps and some forms of the mercury arc. 

4. Types of Lamps. There are a number of types of both 
incandescent and arc lamps. Paragraph 5 gives a tabulation 
of the usual types employed for artificial illumination and 
indicates the character of service for which they are best suited. 
The system of current supply, whether alternating or direct 
current, is also specified. The column headed “lumens per 
watt” gives an indication of the relative efficiency of the various 
lamps, since it shows the amount of light produced per watt 
input. The most efficient lamp is not necessarily the best 
for a particular purpose, since other considerations such as 
quality of light and size of unit must be given proper weight 
when selecting the type of lamp to be used. . The applications 
for which each of the lamps listed in paragraph 5 is best adapted 
will be given in succeeding chapters. 

* Including losses in lamp mechanism. 


PAR. 5 ] TYPES OF LAMPS 3 


6. Types of Electric Lamps. 


Name. 

Service. 

Lumens 

per 

Watt.* 

Suitable for: 

Incandescent Lamps 




1. Carbon filament 

A.C. or D.C. 

3.49 

Interior use. Local lighting.t 

2. Metallized filament 
(Gem) 

A.C. or D.C. 

4.15 

Interior use. Local lighting, t 

3. Tantalum filament 

A.C. or D.C. 

5.54 

Interior use. Local lighting.t 

4. Tungsten. Vacuum 
type. (Mazda B) 

A.C. or D.C. 

10.32 

Interior use. General or local 
lighting. 

5. Tungsten. Gas- 

filled type 
(Mazda C) 

Arc Lamps 

A.C. or D.C. 

21.80 

Interior or general lighting, out¬ 
door use. Street lighting. 

6. Open, carbon arc 

D.C. 

12.30 

Outdoor use. Street lighting. 

7. Enclosed, carbon 

arc 

A.C. or D.C. 

7.35 

Interior or general lighting out¬ 
door use. Large areas. 

8. Flame-arc 

A.C. or D.C. 

28.00 

Interior or general lighting, out¬ 
door use. Large areas. 

9. Metallic-electrodet 

D.C. 

23.40 

Outdoor use. General lighting. 
Large areas. 

10. Mercury-vapor arc 
Low-pressure 
type 

A.C. or D.C. 

13.10 

Interior use. General lighting. 
Large areas. 

11. Mercury-vapor arc 
Quartz-tube 
type 

D.C. 

20.30 

Interior or general lighting, out¬ 
door use. Large areas. 


* For explanation of this term see Chapter 4. Values given are approx¬ 
imate, as they vary slightly with the size of lamp. 

t These lamps are not very commonly used at present. 

X Sometimes called "metallic flame" or "luminous arc." 












CHAPTER 2 


INCANDESCENT LAMPS 

6. General Principles. In Chapter 1 it was pointed out 
that efficient light production by means of incandescent lamps 
is accompanied by a very high temperature of the light-pro¬ 
ducing conductor or filament. This filament, therefore, must 
be composed of a substance which will have a high melting- 
point, and which will not evaporate too rapidly at temperatures 
below the melting-point. Evaporation causes blackening of 
the glass bulb and also the gradual disintegration of .the fila¬ 
ment until it breaks. Evaporation must therefore be reduced 
to a low value for successful operation. Furthermore, the 
resistance of the filament material should be high, so that 
filaments for commercial voltages will have reasonable lengths 
and diameters. 

7. Construction. The filaments of all the commercial in¬ 
candescent lamps now commonly used are enclosed in a glass 
bulb, from which all traces of air are excluded; otherwise 
the filament would rapidly oxidize and be destroyed. In some 
styles of lamp, a high vacuum is produced in the bulb; in others, 
it is filled with nitrogen or a similar gas which will not affect 
the filament. Connection is made to the filament by “lead- 
in-wires’' which are sealed into the glass bulb. The lead-in¬ 
wires are attached to suitable insulated contacts forming the 
base of the lamp. 

8. Life. The life of an incandescent lamp is expressed as 
the number of hours of burning at rated voltage before the 
filament disintegrates and breaks. Since it is impossible to 
build lamps which will all last exactly the same number of 
hours, an average value must be used. This is called the 
rated life. The actual life of individual lamps may be greater 
or less than this. At one time the term useful life was used 
extensively. This was taken as the number of hours burning 
before the candlepower dropped to 80 per cent of the original 

4 


PAR. 9] PERFORMANCE OF LAMPS 5 

* 

value. This method of rating is no longer used to any extent, 
since most modern incandescent lamps do not decrease to 
80 per cent of the original candlepower before they burn 
out. The life of a lamp depends upon the temperature at 
which the filament is operated. A very high temperature 
results in a high efficiency and gives a better color of light, 
but causes rapid blackening of the bulb and disintegration 
of the filament and therefore a short life. A low temperature 
has the opposite effect. The normal operating temperature 
is therefore chosen at such a point as to give the highest effi¬ 
ciency consistent with a reasonable life of the filament. The 
temperature depends upon the voltage applied, a voltage above 
normal giving an increase in temperature and consequently 
higher efficiency, whiter light and shorter life. The reverse 
occurs with voltages below normal. Filaments having a hot 
resistance greater than the cold resistance (such as gem, tan¬ 
talum, and tungsten filaments) are less sensitive to voltage 
changes than the ordinary carbon filament, which has a hot 
resistance less than the cold value.* 

9. Rating. Constant-potential or multiple lamps are rated 
at the normal operating voltage and the watts consumed at 
that voltage. Constant-current or series incandescent lamps 
are rated at the normal operating current and the candle- 
power (mean horizontal) produced at that voltage. 

10. Power Consumption. Incandescent lamps are given 
a commercial rating in watts per candle to facilitate a com¬ 
parison of the efficiency of various types and sizes. Either 
the mean horizontal candlepower f or the mean spherical 
candlepower f is used when stating the power consumption. 
The mean horizontal candlepower was generally used for 
this purpose until recently, but at present the mean spherical 
candlepower is used in rating most types of tungsten lamps. 
The power consumption, expressed in watts per candle, was 
at one time commonly called the “efficiency” of the lamp, 
but this is an incorrect use of the term and should be avoided. 
An incandescent lamp which consumes 400 watts and pro¬ 
duces 445 candlepower has a commercial rating of 400-7-445 
= 0.90 watt per candle : A lamp which consumes 400 watts 
and gives 534 candlepower would have a rating of 0.75 watt 

* See paragraph 34. 

t See paragraph 66. 


6 


INCANDESCENT LAMPS 


[CHAP. 2 


per candle. The second lamp would, of course, be more efficient 
than the other because it requires less power to produce one 
candlepower. The power consumption is also frequently 
specified in terms of the amount of light produced (lumens).* 
Thus we may say that a lamp consuming 100 watts and giving 
a total light output of 1032 lumens has a specific output of 
1032 - 1-100 = 10.32 lumens per watt. While the quantities, 
“lumens per watt” or “watts per candle” do not express the 
true efficiency of the lamp, we can compare the efficiencies of 
two lamps by comparing the power consumption of each. 

Thus the tungsten lamp taking .1.00 watt per candle is 
said to be more efficient than the metallized-filament lamp re¬ 
quiring 2.5 watts per candle. A more correct way to express 
the relative efficiency is to compare the amount of light pro¬ 
duced by one watt. Thus the metallized-filament lamp pro¬ 
duces about 4.15 lumens per watt and the tungsten lamp about 
10 lumens per watt. We can therefore say that the tungsten 
lamp is more than twice as efficient as the metallized-filament 
lamp. 

11. Bases. At the present time, in the United States, 
practically all lamps are provided with the “Edison” screw- 
type base, which has been adopted as standard. This base 
is made in three sizes: mogul, which is 1^ inches in diameter; 
medium, 1 ^ inches in diameter; and small, including candelabra 
bases inch in diameter and miniature bases f inch in diameter. 
Mogul bases are used for street-lighting systems and for the 
larger types of multiple lamps. The medium base is the 
ordinary style used with incandescent lamps for 110- or 220- 
volt circuits, and the small bases are, in general, used for low- 
voltage lamps and special work. In some cases bayonet-type 
bases are used where excessive vibration occurs, as in auto¬ 
mobile lighting systems. For large-size lamps the skirted 
base is used. This is shown in Fig. 3. The skirted base may 
be obtained for both medium and mogul sockets. 

12. Frosted Lamps. Incandescent lamps are sometimes 
frosted when they are to be used without shades and where 
the clear lamp would be objectionable. With a frosted lamp, 
the entire bulb becomes luminous and the filament is not 
visible, thus reducing the intensity of the light. Lamps may 
be frosted by sand-blast or acid, but in either case the surface 

* See Chapter 4 for explanation of this term. 


PAR. 13] 


COLORED LAMPS 


• 7 

of the glass is left rough, making it difficult to keep clean. 
Frosting solutions are available which give a smooth surface, 
but these are not as permanent as the other methods and should 
not be used in locations exposed to the weather. Lamps are 
frequently bowl-frosted, that is, only the lower half of the 
bulb is frosted. These lamps are very commonly used with 
reflectors to shield the lower part of the filament from view. 
The rated life of a frosted lamp is the same as a clear lamp, 
but the candlepower falls off more rapidly. A frosted lamp 
absorbs about 8 per cent of the light produced, and declines 
in candlepower about twice as rapidly as a clear lamp. A 
bowl-frosted lamp absorbs about 5 per cent of the light. 

13. Colored lamps are used for signs and various decorative 
purposes. Lamps made from colored glass are expensive and 
are seldom used. Generally clear lamps are dipped in a color¬ 
ing solution or the bulbs are covered by “color caps.” These 
are small glass globes which fit over the lamp. They are 
used for signs and are more satisfactory than dipped lamps, 
which will not withstand severe weather conditions. Colored 
lamps are much less efficient than the clear lamps because 
a large part of the light produced is absorbed and wasted. A 
special form of gas-filled tungsten lamp is made with a blue 
glass bulb, which is so designed as to correct the color value 
of the light and produce an effect closely resembling daylight. 
This results in a considerable loss of light, but because of the 
very high efficiency of this type of lamp, it is a thoroughly 
practical method to use where true daylight effects are required. 

14. Effect of Alternating Current. Incandescent lamps may 
be used on either direct or alternating current. With the 
exception of the tantalum lamp, there does not seem to be any 
appreciable difference in the life of the lamp when used on 
either system. The life of the tantalum lamp is very much 
less on alternating current. If incandescent lamps are operated 
on a low-frequency alternating-current circuit, there is a notice¬ 
able flickering of the light.* 

15. Color of Light. The color of light produced by an 
incandescent lamp depends upon the temperature. As the 
temperature is increased, the color becomes more nearly white. 
The carbon lamp operates at the lowest temperature, about 
1950° Cent. (3450° Fahr.) and gives a decidedly yellow light. 

* See paragraph 71. 


8 


INCANDESCENT LAMPS 


[chap. 2 


The vacuum-type tungsten filament operates at a temperature 
of about 2100° Cent. (3810° Fahr.) and the gas-filled tungsten 
filament at about 2400° Cent. (4350° Fahr.). The gas-filled 
tungsten lamp therefore produces the whitest light. Incandes¬ 
cent lamps may be arranged in order of their color value as 
follows: carbon, metallized-carbon, tantalum, vacuum-type 
tungsten and gas-filled tungsten. 

16. Distribution of Light. Incandescent lamps used without 
reflectors give the maximum candlepower in a horizontal 
direction, as may be seen by referring to Fig. 20, which gives 
the candlepower measured at various points above and below 
the center of the lamp.* For this particular lamp the hori¬ 
zontal candlepower is 96, while directly below the lamp it 
is only 30 candlepower. The distribution varies with the 
particular arrangement of filament used. 

Carbon-filament Lamps 

t 

17. Construction. This lamp was the earliest commercial 
form of incandescent lamp. It employs a filament of specially 
prepared, dense carbon, operating in a high vacuum. 

18. Applications. Carbon lamps have a very restricted use 
at present because of the many advantages of the more effi¬ 
cient types of incandescent lamps. They are used to a certain 
extent for electric signs, although tungsten lamps are more 
commonly used for this purpose. For 220-volt circuits, car¬ 
bon lamps are sometimes used when small units are required. 
The use of carbon lamps for general illuminating purposes is 
practically discontinued. The advantages of the carbon lamp 
are low first cost and extreme ruggedness. The disadvantages 
are low efficiency and poor quality of light. 

19. Standard Sizes. For 110-volt circuits, tlhe sizes range 
from 20 to 60 watts, and for 220 volts from 35 to 60 watts. 
The power consumption is from 4.2 to 3.0 watts per candle.f 
Sign lamps are made in sizes from 10 to 30 watts with a power 
consumption of from 4.0 to 6.0 watts per candle, f The rated 
life of carbon lamps is about 700 hours. 

* See paragraph 67 for explanation of curves, 
t Mean horizontal candlepower. 


PAR. 20 ] 


SPECIAL KINDS OF LAMPS 


9 


Metallized-filament Lamps 

20. Construction. These lamps use a modified form of 
carbon filament. By a special heat treatment in the electric 
furnace, the ordinary carbon filament is changed into a metal¬ 
lized carbon which has the appearance of very dense graphite. 
The filament is operated in a vacuum the same as an ordinary 
carbon lamp. 

21. Applications. The commercial metallized-filament lamp 
is called the Gem lamp. A few years ago this lamp rapidly 
replaced the ordinary carbon lamp for general illuminating 
purposes, and it has now in turn been replaced to a large extent 
by the tungsten lamp. It is at present used very little for 
general illumination and finds its chief application as a sub¬ 
stitute for small-size carbon lamps where great ruggedness 
and low cost are required. It is especially adapted for portable 
lights in industrial establishments. It cannot compete with 
the tungsten lamp for general illumination unless the cost of 
power is very low—below 1 cent per kilowatt-hour. 

22. Standard Sizes. Table I gives data on the principal 
sizes of Gem lamps which are now produced. The power 
consumption of the Gem lamp is about 2.5 watts per candle,* 
but this varies somewhat, as may be seen from the table. 
The filament has a hot resistance about 2.5 times its cold 
value. 

Tantalum Lamps 

23. Construction. The filament of this type of lamp is 
made from metallic tantalum, and is operated in a vacuum. 
The metal tantalum has a very high melting-point and can 
be operated at a high temperature, but the resistance is low, 
and therefore, for ordinary voltages, the filament length must 
be great and the cross-section very small. The filament is 
wound in a zig-zag manner between special supports. 

24. Applications. A few years ago, the tantalum lamp was 
widely employed for general illuminating purposes, but the 
tungsten lamp, which was introduced shortly afterwards, very 
soon displaced the tantalum lamp, which at the present time 
is little used. Until the development of the wire-drawn tungsten 


* Mean horizontal candlepower. 


10 


INCANDESCENT LAMPS 


[CHAP. 2 


filament the tantalum lamp was more rugged than the tung¬ 
sten lamp, and this was one reason for its extensive use. 

25. Standard Sizes. Tantalum lamps could at one time 
be obtained for both 110-and 220-volt service in sizes ranging 
from 25 watts to 80 watts, but they are no longer manufactured 
in this country. The lamps take about 1.8 watts per candle 
and have a useful life of 800 hours on direct current. On 60- 
cycle current, however, the life is only about 300 hours. 

Tungsten Lamps—Vacuum Type 


26. Construction. The filament of tungsten lamps is com¬ 
posed of pure metallic tungsten. ' Originally the filament was 
made from a paste containing powdered tungsten, but at 

present all the filaments are of the drawn- 
wire type. These filaments are made by 
drawing tungsten through dies, using dia¬ 
mond dies for the finishing. Filaments 
made from powdered tungsten were ex¬ 
tremely fragile and consisted of several 
hairpin loops connected in series. The 
wire-drawn filament is composed of one 
continuous length of tungsten wire looped 
around suitable supports and is very strong. 
Fig. 1 shows the ordinary type of tungsten 
lamp. For some purposes it is necessary 
to concentrate the filament. To accomplish 

this, the tungsten wire is wou’nd in the 
Fig. 1. Tungsten f orm G f 

a very small helix or single-layer 
Lamp, 'Vacuum co j^ thus reducing the space occupied by 

the filament (Fig. 2). This type of con- 
(40-watt,^Mazda B.) gtruction is sometimes used for vacuum 

type lamps and is always used for gas-filled 
tungsten lamps. The tungsten wire, after being drawn through 
dies and before it is heated in the bulb, has a tensile strength 
greater than steel and can be readily bent or wound into spirals. 
After the lamp has been operated for a short time, the wire¬ 
drawn filament becomes more brittle, but it is still much su¬ 
perior to the old type of filament, and has sufficient strength 
to be used where there is considerable vibration, as in the 
lighting of electric cars. In order to prevent blackening of 
the bulb, due to evaporation from the filament, a special chem- 





















PAR. 27 ] TUNGSTEN LAMPS—VACUUM TYPE 


11 


ical, called a getter,* is placed in the bulb. This combines 
with black particles given off from the filament and changes 
them to a transparent substance which does not cut off the 
light when deposited on the bulb. A very high vacuum is 
used with this type of lamp. The diameter of the filament 
of the tungsten lamp depends upon the current to be carried, 
and varies from about 0.0006 inch for the 10-watt, 110-volt 
size, which takes 0.091 ampere, to about 0.020 inch for a series 
lamp taking 20 amperes. The length of the filament depends 
upon the voltage. The filament of a low-voltage lamp of 
a given watt rating is larger in diameter and shorter than 
the filament for a high-voltage lamp of the same rating in 
watts. This is apparent when we recognize that the low- 
voltage lamp must carry a larger current (for the same watts) 
than the high-voltage lamp. 

27. Applications. The tungsten lamp is now the most widely 
used of all the types of incandescent lamps, about 80 per cent 
of all lamps used at present f being of this type. It is adapted 
for any service where incandescent lamps are required, except 
when they are subjected to very rough usage, such as portable 
lamps. The principal field of usefulness of the vacuum-type 
lamp for general illumination purposes is in sizes below 100 
watts, the gas-filled lamp being generally preferable for larger 
sizes. Vacuum-type tungsten lamps are also widely used for 
railway car lighting, electric signs, automobile lighting, and 
for flash-lamps. The wire-drawn filament is strong enough 
to allow the successful use of 110-volt tungsten lamps as small 
as 23 watts in the lighting of electric cars. Concentrated 
filament lamps, with reflectors, are used for stereopticon and 
small moving-picture machines, for automobile, locomotive and 
street-railway headlights and for flood lighting { of bill boards, 
building fronts, etc. The advantages of the tungsten, vac¬ 
uum-type lamp are high efficiency, excellent color of light and 
adaptability for various classes of service. Compared with 
the Gem lamp, the tungsten lamp is somewhat more expensive, 
but its superiority in other respects justifies its use except for 
small units where power is very cheap; for example, below 
1 cent per kilowatt hour or where the lamp would be sub¬ 
jected to very rough usage. 

* Or “vacuum getter.” t 1916. 

t See paragraph 142. 


12 


INCANDESCENT LAMPS 


[CHAP. 2 


28. Standard Sizes. The commercial, vacuum-type tung¬ 
sten lamps are known generally as ‘‘Mazda” or “Mazda B” 
lamps. These names are applied to the lamps made by most 
of the lamp manufacturers in this country, although there are 
a few manufacturers, producing lamps under different patents, 
who use other names. Mazda lamps can now be obtained for 

any voltage or service for which in¬ 
candescent lighting is suited. Table 2 
gives data on the commercial sizes of 
Mazda B lamps for multiple service 
and general illuminating purposes. 
While the lamps can be supplied for 
various voltages from 105 to 125 volts 
and from 220 to 250 volts, the exact 
operating voltage must, of course, be 
specified when ordering. The rated 
life of the ordinary multiple lamps is 
1000 hours for most sizes. Stereopti- 
con and headlight lamps have a short 
life varying from 220 to 500 hours. 
Sign lamps have a life of 2000 hours. 
The power consumption of vacuum- 
type tungsten lamps is at present 
about 1.28 watts per mean spherical 
candle.* There has been a continued 
improvement in this respect during the last few years, due to 
improvements in manufacture. The color of the light is 
entirely satisfactory for all general illuminating purposes, even 
where practically correct color values are important. 

Tungsten Lamps—Gas-filled Type 

29. Construction. The gas-filled type of lamp employs 
a wire-drawn tungsten filament operated in a glass bulb con¬ 
taining nitrogen or similar gas. The pressure of the gas when 
the lamp is lighted is about equal to the air pressure outside 
the bulb, so that there is no danger of the lamp exploding. 
As has been previously explained, the working temperature 
of an incandescent filament is limited by the tendency to 
rapid evaporation and blackening of the bulb if the temper- 

* 1.0 watt per mean horizontal candle. 



Fig. 2. —Enlarged View of 
Coil Filament for Tung¬ 
sten Lamp. 

View of a filament for a series 
type, 250-cp. lamp. For mul¬ 
tiple lamps a longer filament of 
the same form is used. (Na¬ 
tional Lamp Works of G. E. 
Co.) 





PAR. 29 ] TUNGSTEN LAMPS—GAS-FILLED TYPE 


13 


atiire is made too high. The rate of evaporation increases 
rapidly as the temperature approaches the melting-point of 
the filament. This effect may be compared to the vaporizing 
of water at temperatures slightly below the boiling-point. 
If water is heated in a vacuum, it is well known that the boiling- 
point is very much reduced and in the same way the vapor¬ 
izing effect will take place at a lower temperature. The use 
of a vacuum in an incandescent lamp, therefore, increases 
the amount of vaporization of the filament at a given tem¬ 
perature. If the filament is operated at ordinary atmospheric 
pressure, the temperature can be raised considerably higher 
before there is excessive evaporation. The filament could 
not be operated in the air, as it would oxidize or burn. If, 
however, the bulb is filled with nitrogen or a similar gas, there 
is no tendency to oxidize and the filament can be run at a higher 
temperature than for the vacuum-type of lamp. The presence 
of gas in the bulb results, however, 
in a cooling of the filament by con¬ 
vection currents, and to counteract 
this as much as possible, a concen¬ 
trated filament is used.* The close¬ 
ness of the coils results in a mutual 
heating effect and exposes a mini¬ 
mum amount of the filament to the 
cooling action of the gas. Fig. 3 
illustrates the gas-filled type of 
lamp. It will be noted that this 
lamp is provided with a long neck, 
which assists in keeping the base, 
and socket cool. The mica baffle 
retards the circulation of the gas and 
also helps to keep these parts cool, 

Lamps of the smaller sizes are made 
with straight sides without the mica baffie and have the same 
general appearance as the vacuum lamps, Fig. 1. This style 
is most commonly used for street lighting service where there is 
thorough ventilation. Gas-filled lamps are most successful in the 
larger sizes, because small sizes of filament expose relatively more 
surface to the cooling effect of the gas. The vacuum-type 
lamp can be burned in any position, but the standard position 

* See paragraph 26. 



Fig. 3. —Gas-filled Tung¬ 
sten Lamp. 

(750-watt, Mazda C.) (| scale). 

































14 


INCANDESCENT LAMPS 


[chap. 2 


for the gas-filled lamp is vertical, with the tip of the lamp 
down.* Lamps can be provided on special order which may 
be burned with the tip up. With the lamp burning in the 
normal position (tip down) the blackening of the bulb takes 
place principally in the upper part where it has little effect 
in shutting off the light. 

30. Applications. Gas-filled tungsten lamps are particularly 
well adapted for interior illumination of large areas, for out¬ 
door illumination of various kinds, and for street lighting, 
where they have already replaced large numbers of arc lamps. 
Because of the concentrated form of filament, they can be 
used very successfully with reflectors, for locomotive and 
street-railway headlights and for stereopticons and small mov¬ 
ing-picture machines. Lamps with special glass bulbs are used 
in photographic work, and for show windows. The lamps are 
not suited for illuminating small rooms or rooms with low 
ceilings, because of the high candlepower of the commercial 
sizes. They are a very strong competitor of the various modern 
arc lamps and are rapidly replacing them for many purposes. 

31. Standard Sizes. The commercial gas-filled tungsten 
lamps are generally known as “ Mazda C” lamps and some¬ 
times as nitrogen lamps. Table 3 gives data on the commer¬ 
cial sizes of these lamps for multiple service and general illu¬ 
minating purposes. Gas-filled lamps for series, street-lighting 
circuits are manufactured for 5.5, 6.6, 7.5, 15.0, and 20.0 amperes 
in sizes ranging from 60 to 1000 candlepower, although the 
complete range of sizes is not made for each current rating. 
The rated life of the ordinary multiple lamps is 1000 hours. 
Series lamps have a rated life of 1350 hours. Headlight lamps 
have a life of 300 to 500 hours, depending upon the size. The 
power consumption is about 0.74 watt per candle | for mul¬ 
tiple lamps and reaches 0.57 watt per candlef for large series 
type lamps. The manufacturers do not recommend the frost¬ 
ing of Mazda C lamps because of the additional heating. 

32. Color of Light. The light from the gas-filled tungsten 
lamp is whiter than that from the vacuum-type lamp and 
more nearly approaches daylight than any other incandescent 
lamp. It is excelled in this respect only by certain types of 

* Except for the 75- Jind 100-watt sizes, which can be burned in any position. 

f Mean spherical candlepower. For mean horizontal candlepower, the 
values are 0.59 and 0.45 respectively. 


PAR. 33 ] 


OPERATING CHARACTERISTICS 


15 


arc lamps. The light is suitable for general illuminating pur¬ 
poses, even where color values are important. For very 
exacting service, such as color matching in paint and dye 
works, and for show windows, lamps with blue glass bulbs 
are used to reproduce daylight effects. These lamps are 
called daylight Mazdas and are not as efficient as the clear 
glass lamps. 

33. Heating Effects. Over 95 per cent of the power supplied 
to an incandescent lamp is given off in the form of heat. With 
large tungsten lamps, the amount of heat is so great as to 
necessitate careful ventilation. While the heat produced by 
either the vacuum or gas-filled lamp is practically the same for 
the same watt rating, the distribution of the heat is entirely 
different in the two styles. With the vacuum-type lamp, 
the heat is uniformly distributed over the whole bulb, but 
with the gas-filled lamp, burning tip down (as is usual), the 
gas carries a large proportion of the heat up to the neck and base 
of the lamp. These parts are consequently hotter than other 
parts of the lamp, with resulting high temperatures for the lamp 
socket and reflector. It is therefore advisable to avoid sockets 
which employ wax compounds or fibre as a part of the insula¬ 
tion. The use of rubber-covered wire for connecting the sockets 
should in general be avoided. Where lamps are exposed to the 
weather, it is necessary to shield the lamp bulb so that rain and 
sleet cannot come in contact with the upper part of the lamp. 
This is particularly necessary with the large-size units where the 
bulbs are so hot that there is danger of cracking the glass if 
it becomes wet.* 

34. Effects of Voltage Variation. Tungsten lamps of all 
kinds give their rated candlepower and consume their rated 
watts only at the particular voltage for which they are designed. 
In any installation, the rated lamp voltage should of course 
be the average operating voltage of the system, measured at 
the lamps. The effect of a change in voltage has already been 
explained in paragraph 8. With tungsten lamps, the changes 
are not as great as for carbon-filament lamps. With an in¬ 
crease of 1 per cent in voltage, the candlepower of tungsten 
lamps increases about 3.6 per cent and the life is decreased about 
13 per cent. A decrease in voltage has the opposite effect. 

35. Overshooting. The resistance of all tungsten lamp fila- 

* See paragraph 86. 


16 


INCANDESCENT LAMPS 


[CHAP. 2 


ments, when cold, is only about one-twelfth the hot resistance. 
When the lamp is first thrown on the circuit, this causes a sudden 
rush of current amounting to about five to eight times normal 
at the instant when the switch is closed. This is less than the 
value calculated from the cold resistance (about twelve times 
normal) due to the effect of the resistance and reactance of the 
circuit and the heating of the filament. The current drops very 
rapidly to practically normal value at the end of 0.02 second. 
The excess current lasts for so short a time that it would not 
blow fuses, but ordinary overload circuit breakers, if set close 
to the normal value of lamp load, would trip. This can be 
avoided, however, by introducing a small time-element in the 
tripping of the breaker. This current rush has also been known 
to melt the solder at the base of the lamp and fuse it into the 
receptacle. This occurs only with the larger sizes of lamps, 
when a loose contact is made with the circuit, due to vibration 
partly unscrewing the lamp. When a tungsten lamp is con¬ 
nected to a constant-potential circuit, the candlepower rises 
to its normal value much more quickly than a carbon lamp. 
This causes an effect which is sometimes called “ over-shoot¬ 
ing,” because the candlepower seems to be higher than normal 
for an instant. Apparently, however, this is an illusion due 
to the very rapid rise of candlepower.* This effect is of value 
when using tungsten lamps for the flashing type of signs, as 
it allows more rapid flashing effects. 

t 

* Electrical World, May 25, 1912, 


CHAPTER 3 


ARC LAMPS 

36. General Principles. When the two terminals of an elec¬ 
tric circuit are brought together with a loose contact and then 
separated slightly, the heat produced at the moment of sepa¬ 
ration vaporizes a portion of the terminals and forms an electric 
arc. The temperature of the electric arc is about 4000° Cent. 
(7230° Fahr.), and therefore the terminals or electrodes must 
be capable of withstanding a high temperature without melt¬ 
ing. Until recently all arc lamps employed carbon electrodes. 
At present, however, various metals and other substances are 
used. An arc between pure carbon electrodes is a pale violet 
in color and gives off very little light. The principal sources 
of light are the spots on the electrodes where the arc touches. 
With d.c. arcs, more light is given off by the tip of the positive 
carbon than by the negative, and it is for this reason that an 
ordinary d.c. arc lamp should always be operated with the top 
carbon positive, so that most of the light will be directed down¬ 
ward in a useful direction. With an a.c. arc, there is not 
this difference. In the new types of lamps the arc itself is 
rendered luminous by the use of various chemicals in the elec¬ 
trodes with a resulting gain in efficiency. The flame-arc lamps 
are of this type. 

37. Construction. Arc lamps consist essentially of a pair 
of electrodes of carbon or other suitable material, arranged 
to be brought into contact and then separated a short distance. 
Since the terminals must be maintained a fixed distance apart, 
some kind of mechanism is required to constantly “ feed ” 
the electrodes together as they are consumed, as well as to 
bring them together to “ strike ” or start the arc. These 
operations are accomplished by solenoid magnets, connected 
in the lamp circuit, which operate a suitable clutch mechanism 
attached to one or both of the electrodes. Vapor-tube lamps, 

17 


18 


ARC LAMPS 


[chap. 3 


which are also classed as arc lamps, do not require a feeding 
mechanism, but a device for starting must be employed in some 
cases. If arc lamps are used on series or constant-current 
circuits, they must also be provided with a suitable automatic 
cutout to short circuit the lamp and thus keep the main circuit 
closed if the carbons break, or if the lamp circuit is opened by 
the carbons being entirely consumed. For lamps used on mul¬ 
tiple or constant-potential circuits no automatic cutout mechan¬ 
ism is required, but the lamp must have, in addition to the carbon 
feeding mechanism, a steadying resistance. This is required 
because of the unstable nature of the arc and the tendency 
of the current to increase with a decrease in voltage. The 
proper steadying action is produced by a resistance in the case 
of d.c. lamps, and by a reactance coil in the case of a.c. lamps, 
the latter being used because less power is lost than with 
a resistance. Some types of arc lamps have the arc freely 
exposed to the air, but the more modern lamps have the arc 
enclosed in a glass globe which is nearly airtight. 

38. Rating. Constant-potential or multiple arc lamps are 
rated at the normal operating voltage and the current which 
the lamp requires at that voltage. Most types of these lamps 
may be adjusted to take a current slightly above or below 
normal. The lamps may also be adjusted to operate on any 
voltage within 10 per cent above or below normal. Constant- 
current or series lamps are rated at 'the normal operating cur¬ 
rent required. They can usually be adjusted to operate on 
currents slightly greater or less than the normal value. At 
one time arc lamps were rated in candlepower, but this rating 
is no longer used, since it is meaningless. 

39. Power Consumption. The power consumption of arc 
lamps is specified commercially in watts per mean lower hemi¬ 
spherical candlepower.* This method of rating allows a com¬ 
parison of the relative efficiency of two types of arc lamps, but 
should not be used for comparing arc lamps with incandescent 
lamps rated in watts per mean horizontal candlepower.* If 
the watts per mean spherical candlepower are used for com¬ 
parison a true measure of the relative efficiencies is obtained, 
since the total light output is considered in measuring the 
candlepower. For this reason, statements of power consump¬ 
tion for both arc and incandescent lamps are now commonly 

* See Chapter 4 for explanation of this term. 


PAR. 40 ] 


OPEN CARBON ARC LAMPS 


19 


made in watts per mean spherical candlepower. We can also 
accurately compare the efficiency of two lamps by stating the 
output of light in lumens per watt. The power consumption 
is always based on the total watts consumed by the lamp, 
thus including the losses in the feeding mechanism and the 
steadying resistance. 

40. Systems of Power Supply. Nearly all kinds of arc lamps 
can be built for operation on either a.c. or d.c. systems, but 
a lamp adapted for one system or for a particular frequency 
cannot be used on a different system. A.c. arc lamps, in general, 
are not satisfactory for use at frequencies less than 40 cycles, 
because of the flickering of the arc. The flame-carbon lamp, 
however, has been successfully used on 25 cycles. Variations 
of voltage are no-t of as much importance as with incandescent 
lamps. 

Open Carbon Arc Lamps 

41. Construction. The open-arc lamp, employing two car¬ 
bon pencils as electrodes, was the first commercial type of lamp 
to be used, and was at one time the most efficient form of 
illuminant. As the name indicates, the arc produced between 
the electrodes is exposed to the air. Both d.c. and a.c. open- 
arc lamps have been used, but the d.c. lamp is more common. 

42. Applications. The d.c. series, open-arc lamp has been 
very extensively used in the United States for street lighting 
and to a limited extent for interior lighting, although in the 
latter case it is not very satisfactory because of the danger 
from the high-voltage circuit and also from escaping sparks. 
The lamp has now been almost entirely replaced by other types 
of lamps. The d.c., multiple, open-arc lamp has never been 
extensively used in this country for general illumination. It is, 
however, used extensively for search lights, stage ighting, 
and for stereopticons and moving-picture machines. Neither 
the series nor multiple a.c. open-arc lamp has been used exten¬ 
sively in this country, principally because of the noise and poor 
distribution of the light. Alternating current is used to a certain 
extent for hand-feed lamps for projectors where direct current 
is not available, but the lamps are not as satisfactory as the 
d.c. lamps. For this reason, especially for moving-picture 
machines, a converting device is frequently installed to change 
from alternating to direct current. The disadvantages of open 


20 


ARC LAMPS 


[CHAP. 3 


arcs for general illuminating purposes are unsteady light, 
strong shadows, frequent trimming, noise and fire hazard. 

43. Standard Sizes. The d.c. series, open-arc lamps com¬ 
monly used for street lighting have the following ratings: 


Amperes. 

Volts Arc. 

Voltage at 
Terminals. 

Candle- 

power.* 

Watts per 
Candle.* 

6.6 

48 

50 

395 

0.82 

9.6 

48 

50 

690 

0.71 


* Mean lower, hemispherical. 


The consumption per mean spherical candlepower is about 1.25 
watts for the 6.6-ampere lamp and 1.02 for the 9.6-ampere arc. 
As the life of a 12-in. carbon electrode in these lamps is only 
from seven to eight hours, usually a lamp is provided with a 
second pair of electrodes which can be cut into circuit when 
the first pair has been consumed, thus reducing the amount of 
attention required. 

44. Distribution and Color of Light. The light from the 
d.c. arc lamp with positive carbon at the top is principally 
directed below the horizontal. The maximum candlepower 
of the 6.6-ampere lamp is about 720 at an angle of 45° with 
the horizontal. For the 9.6-ampere arc the maximum candle- 
power is 1250 at the same angle. Due to the short arc (about 
| in. long), the lower carbon partially shields the light from the 
upper carbon and this produces a large dark area directly 
below the lamp. The a.c. open arc gives a much larger propor¬ 
tion of light above the horizontal and is always used with a 
reflector. The color of light produced by the open arc is a 
bluish-white. 


Enclosed Carbon Arc Lamps 

45. Construction. The enclosed arc lamp differs from the 
open type in having the arc enclosed in a glass globe, called the 
inner globe, which is made nearly airtight. This globe is closed 
at the top by a metal cap or gas check (Fig. 4) designed to admit 
only enough air to consume the carbon vapor formed by the 














PAR. 45 ] ENCLOSED CARBON ARC LAMPS 


21 


» 



Fig. 4.—D.C. Enclosed Arc Lamps. 


1. Inner globe. 2. Gas check. 3. Series coils for operating clutch on upper 
carbon, adjusting length of arc and thereby keeping current steady. 4. Steady¬ 
ing resistance, adjustable by contact 5. 6. Contacts for changing current taken 

by lamp. 7. Cutout switch. 8. Clutch. 9. Series coil to regulate arc length. 
10. Shunt coil across arc, balancing pull of series coil (9) and keeping voltage 
across arc constant, 11, Dash-pot for steadying action of magnets. 12. Cut¬ 
out. 








































































































































































22 


i 


ARC LAMPS 


[CHAP. 3 


arc. By this means the life of the electrodes is greatly in¬ 
creased. If the air supply is restricted too much the carbon 
vapor will deposit on the globe and shut off the light; on the 
other hand, an excess of air decreases the electrode life. En¬ 
closing the arc, and thereby shielding it from air currents, makes 
possible a longer arc (about f in.), a higher voltage across the 
arc, and the use of a smaller current. With the open type, 
the arc voltage is about 48, while with the enclosed arc the 
voltage is about 72. The electrodes in the d.c. enclosed lamp 
are pure carbon. For the a.c. lamp, better operation is secured 
when one of the carbons is cored. Cored carbons have a small 
hole running the entire length and containing certain chem¬ 
icals which vaporize easily and assist in keeping the arc steady. 
Fig. 4 gives diagrammatic views of enclosed, d.c. lamps. The 
arrangement for a.c. lamps is similar. Fig. 4a does not show 
the clutch and dash-pot for this lamp, but they are similar to 
those shown in Fig. 46. 

46. Applications. The series lamps are used principally for 
street lighting and other outdoor applications, because of the 
hazard when using high-voltage circuits in buildings. The 
d.c. series lamp is more commonly used than the a.c. lamp 
because of a better downward distribution of light and the 
absence of noise. The a.c. lamps are usually operated from 
60-cycle circuits. The multiple lamps are used for both street¬ 
lighting and interior illumination. Their use in street lighting 
is confined to localities which have low-voltage, constant- 
potential mains as in the business portions of large cities. These 
lamps' are now being rapidly replaced by gas-filled tungsten 
lamps, which are frequently mounted in the casing and globe 
formerly used for the arc lamp. The indoor type of multiple 
lamp has been widely used for lighting large areas in factories 
and for store lighting. They are being replaced in more 
modern installations, however, either by large tungsten units 
or by flame-arc lamps. The enclosed arc lamps are steadier 
than the open arcs, but the light is bluer. The efficiency of 
the enclosed arc lamp is lower than the open arc because of the 
lower temperature of the electrode tips and the absorption of 
light by the enclosing globes. The advantages over the open 
arc are longer electrode life and better light distribution due 
to the use of a longer arc. The a.c. lamps are noisy and do 
not give as high efficiency nor as good light distribution as the 


PAR. 47 ] 


ENCLOSED CARBON ARC LAMPS 


23 


d.c. lamps. Besides their use for general illumination purposes, 
enclosed arc lamps are employed in headlights for locomotives 
and electric cars. 

47. Standard Sizes. Typical performance data for enclosed 
arc lamps are given in Table 4. This includes the principal sizes 
now manufactured. A.c. arc lamps are manufactured for 
operation on 40, 60, and 133 cycles, but 60-cycle lamps are the 
most commonly used. The 220-volt d.c. multiple lamp is not 
as efficient nor as satisfactory as the 110-volt lamp because of 
the longer arc and the smaller current used. It is possible to 
obtain d.c. multiple-series lamps for use on constant-poten- 



30 30 

Fig. 5.—Light Distribution of Enclosed Arc Lamps. 

" Curve A, 6-ampere d.c. multiple lamp, no reflector, opal inner globe and clear 
outer globe. Curve B, 7.5-ampere a.c. series lamp, no reflector, opal inner 
globe and clear outer globe. Curve C, same as B with porcelain enamel reflector. 


tial circuits operating at 220 and above. The lamps are 
similar to the 110-volt multiple lamp, except the operating 
mechanism, which inserts a resistance in the circuit when the 
lamp strikes the arc. This maintains the current at the proper 
value for the other lamps in series. Each lamp is adjusted to 
take 110 volts. A.c. lamps are operated on systems having 
voltages higher than 110 by using a small transformer or 
compensator to step down the line voltage. Ordinary enclosed 
arc lamps show a falling off of light during the life of a set of 
electrodes, due to a deposit which forms on the inner globe. 
Tesfrs * show that for one hundred hours burning the loss is 

* Mathews, Proceedings National Electric Light Assn., 1901, p. 296. 











24 ARC LAMPS [chap. 3 

5 per cent for the best grade of carbons and 30 per cent for low- 
grade carbons. 

48. Distribution and Color of Light. The distribution of 
light in various directions is shown in Fig. 5. With the d.c. 
lamp, without reflector, the largest proportion of the light is 
directed below the horizontal. The maximum candlepower, 
about 420 cp. for the lamp illustrated, is produced at an angle 
of about 30° below the horizontal. With the a.c. arc lamp 
a large amount of the light is directed above the horizontal. 
By the use of reflectors, however, the light can be directed 
in a downward direction. It is customary, in general, to use 
the lamps without reflectors for outdoor service and to use 
small reflectors for interior lighting, although the need of re¬ 
flectors for d.c. lamps is not as great as for the a.c. type. The 
light from the enclosed arc is bluer in color than the open 
arc owing to the greater length of the enclosed arc. The light 
is not very satisfactory for matching colors. By the use 
of opal enclosing globes, the color of the light is somewhat 
improved at the expense of lower efficiency. 

49. The Intensified Arc. This lamp is a modified form of 
enclosed arc using small-diameter carbons. Lamps have been 
developed for d.c. and a.c. multiple circuits, but the d.c. type 
is most commonly used. The commercial d.c. lamp operates 
at 110 volts and requires 5 amperes. The lower, negative 
carbon is f in. in diameter and the upper electrode consists 
of two £-in. diameter carbons. Because of the small size of 
the positive carbons, they become intensely hot at the tip,. 
thus giving an increased light and resulting in a higher efficiency. 
The performance of this lamp is given in Table 4. The light is 
whiter than the ordinary enclosed arc. The lamp has been 
used for interior illumination of stores, offices and for similar 
applications, but is not used to any extent at the present time, 
since tungsten lamps are more satisfactory. 

Flame-arc Lamps 

50. Construction. Flame-arc lamps employ carbons im¬ 
pregnated with chemicals which render the arc luminous. 
Most of the light is produced by the arc itself, very little light 
coming from the electrodes. The lamp is sometimes called 
the flaming-arc lamp. During the operation of the lamp, a 
considerable amount of duet is produced and special provision 


PAR. 50 ] 


FLAME-ARC LAMPS 


25 


is made to remove this from the region of the arc, to prevent 
its accumulation on the globe and the consequent shutting off 
of the light. The first lamps of this type were open arcs and 
were not satisfactory, owing to the short life of the carbons 
(about fifteen hours), the objectionable 
fumes, and the flickering of the arc. 

Modern lamps are now made of the 
enclosed type giving an electrode life 
of about one hundred hours, and greater 
steadiness of the light. Fig. 6 illus¬ 
trates the type of lamp made by one 
manufacturer. The carbons are en¬ 
closed in an inner and an outer 
globe (Fig. 6a). The inner globe con- 




Fig. 6.—D.C. Multiple Flame-arc Lamp. 

1. Condenser for removing fumes and dust from arc chamber. 2. Absorbing 
material for removing glass-etching gases. 3. Economizer for confining arc. 
4. Inner globe. 5. Outer globe. Arrows show circulation of air carrying fumes 
to condensing chamber. 6. Series coil for operating clutch (9) on upper carbon, 
adjusting the arc and keeping the current steady. 7. Steadying resistance. 
8. Blow-coil to keep arc central. 9. Clutch. 10. Dash-pot for steadying the 
action of the series coil (6). 


nects with a “ condensing chamber ” of metal. The fumes 
from the arc are carried by air currents into this chamber, and 
are condensed, since this is cooler than the globe. There are 
certain gases formed which attack the glass and these are 
removed by the absorbing material (2) in the condensing 







































































26 


ARC LAMPS 


[CHAP. 3 


chamber. It has been found that higher efficiency and greater 
steadiness of light is obtained if the arc is partly confined. 
The “economizer” (Fig. 6) accomplishes this. It is therefore 
necessary to keep the arc at a fixed distance below the econo¬ 
mizer, and this is done by feeding both carbons instead of the 
upper one only as is usual with enclosed arc lamps. Fig. 66 
shows the feeding mechanism and connections for a d.c. multiple 
lamp. A similar arrangement is employed for the lamps oper¬ 
ating on the other kinds of circuits. In order to neutralize 
the magnetic effect of the current flowing in the rod support¬ 
ing the lower carbon, a blow-coil (8) is used. This prevents 
uneven burning of the carbons. The lamps employ a long arc, 
about 1.75 in. for 110-volt service. Owing to the presence 
of chemicals in the carbons, there is sometimes difficulty with 
these lamps due to “ slagging ” or the formation of glass-like 


Candle Power Candle Power 

1600 1200 800 400 400 300 1200 1600 1600 1200 800 400 400 800 1200 1CU0 



a. A.C. 7.5-ampere multiple lamp. b. D.C. 6.5-ampere multiple lamp. 

Fig. 7. —Light Distribution of Flame-arc Lamps without Reflectors. 

Curve A, lamps equipped with clear outer and inner globes. Curve B, lamps 
equipped with translucent outer and clear inner globes. White-light carbons 
used. (Westinghouse Elec. & Mfg. Co.) 


knobs on the tips of the electrodes. This slag is a non-con¬ 
ductor and sometimes opens the circuit and prevents the lamp 
from starting. With proper electrodes, however, the slag 
which forms is easily knocked off by the blow when the elec¬ 
trodes come together. 

61. Applications. The flame-arc lamps are especially adapted 
for the illumination of large areas, where the lamp can be 
mounted at a considerable height. Because of their high candle- 
power, they should never be used for small rooms or where they 
are in the direct line of vision. They are specially suited for 
lighting smoky or dusty places such as foundries, blacksmith 
shops, and railway yards, and have been used extensively for 


















PAR. 52] 


FLAME-ARC LAMPS 


27 


street lighting. The lamps are used on both direct and alter¬ 
nating current and for series and multiple circuits. A.c. lamps 
are usually operated on 60-cycle circuits, but they are also made 
for 25 cycles. The gas-filled tungsten lamp is a serious com¬ 
petitor of the flame-arc lamp and the tendency is at present 
to use the tungsten lamp for many places where a few years 
ago the flame-arc would have been employed. 

52. Standard Sizes. The commercial sizes of enclosed or 
long-burning flame-arc lamps vary somewhat with different 
manufacturers. Table 5 gives data for the common sizes. 
By providing small transformers or compensators, the series 
a.c. lamp may be operated on circuits having different values 
of current. The a.c. multiple lamp is adapted for 220-volt 
circuits by using a small transformer mounted inside the lamp. 
Lamps for 25 cycles cost somewhat more than 60-cycle lamps. 
The electrodes are usually 14 in. long and f in. in diameter. 
For enclosed flame-arc lamps, the decrease in the lower mean 
horizontal candlepower during 100 hours’ burning is about 
20 per cent. 

63. Distribution and Color of Light. The distribution of 
light from the flame-arc without reflector depends upon the 
kind of enclosing globe used. 

With clear outer and inner 
globes such as would be 
used out doors, the maximum 
candlepower for the d.c. 

(10-ampere) lamp is about 
1900 at an angle of 11 0 below 
the horizontal. For the a.c. 

(10-ampere) lamp, with clear 
globes and no reflector, the 
maximum candlepower is 
about 1700 at 8° below the 
horizontal. The distribution 
of light for d.c. and a.c. lamps 
is shown in Fig. 7. It will be 
noted that the maximum candlepower is at practically the 
same angle (about 30° below horizontal) for both d.c. and a.c. 
lamps. The use of opal diffusing globes is generally necessary 
for interior illumination, although the efficiency is thereby 
reduced about 20 per cent. For industrial purposes, reflec- 



Fig. 8. —Light Distribution of 
Flame-arc Lamps with In¬ 
dustrial Reflectors. 

Curve A, A.C. 7.5-ampere multiple 
lamp, clear inner globe. Curve B, D.C. 
6.5-ampere multiple lamp, clear inner 
globe. White-light carbons used. (West- 
inghouse Electric & Mfg. Co.) 






28 


arc lamps 


[chap. 3 


tors are sometimes employed to concentrate the light. These 
reflectors produce a distribution about as shown in Fig. 8. 
The light produced by the flame-arc may be made either white 
or yellow, depending upon the kind of electrodes used. The 
curves already discussed were made from lamps using white- 
light carbons. Yellow-light carbons give about 25 per cent 
more light. White-light carbons are usually employed for 
street lighting and interior illumination. For railway yards 
and smoky or dusty places, the yellow-light carbons are pre¬ 
ferred. 

Metallic-electrode Arc Lamps 

54. Construction. This type of lamp is sometimes called 
the magnetite lamp, because one of the electrodes is composed 
of an ore of iron called magnetite. The lamps are in many 
respects different from either the enclosed arc or the flame-arc. 
No carbon is used in the electrodes of the lamp and the light 
comes almost entirely from the arc itself. The lamp is used 
only on direct current. The positive electrode is a composite 
block of copper and iron, the form varying with the different 
manufacturers. The negative electrode is composed of a mix¬ 
ture of powdered iron oxide or magnetite and several light- 
producing oxides, all moulded into the form of a rod. The 
material is a non-conductor when cold, so the rod is either 
enclosed in an iron tube or has an iron wire moulded into the 
center to carry the current. The positive electrode is not 
consumed rapidly and has a life of several thousand hours 
for one type of lamp. For another make, this terminal is 
smaller and is replaced at each trimming of the lamp. In 
the Westinghouse lamps, the positive terminal is at the bottom, 
while in the General Electric lamp, this terminal is at the 
top. Each company claims advantages for the arrangement 
adopted. Because of the fumes and soot produced by the lamp, 
special arrangements are made to ventilate the arc space and 
carry these fumes out of the lamp. The arc is enclosed in a 
single large globe, generally of clear glass. Fig. 9 shows the 
arrangement used by one manufacturer. The feeding system 
is similar to that used in the enclosed arc lamp. 

66. Applications. The lamp is principally used on series 
or constant-current circuits for street lighting and is now the 
standard lamp for this purpose. It is much more extensively 


PAR. 5G] METALLIC-ELECTRODE ARC LAMPS 


29 


used for this work than either the flame-arc or the high candle- 
power tungsten units, although the latter are rapidly gaining 
in favor. The metallic-electrode arc is also used in head¬ 
lights for interurban cars. The lamp is also made for d.c. 
multiple circuits and is used for the illumination of parks, 
mills, foundries, machine shops, train-sheds, freight yards and 
similar places. The lamps arranged for pendant burning, 
as shown in Fig. 9, are commonly used for street lighting. 



a. Sectional view showing refractor. b. Connections. 

Fig. 9. —Series Metallic-electrode Arc Lamp. 

When lamp is not in service electrodes are separated. When lamp is com 
nected to circuit, current flows from + through starting resistance (5), contacts 
(4) and starting magnet (1). This raises the lower electrode by means of clutch 
(2) and contact is made with upper electrode. Current then flows through series 
magnet (3), which opens contacts (4), thus separating electrodes and striking arc. 
The shunt magnet (6) is now energised through the starting circuit. If the arc 
becomes too long the voltage rises, the shunt magnet closes contacts (4) and the 
electrodes are brought together again, the clutch taking a different position on 
the electrode rod. 


An ornamental type of lamp is also made for mounting at the 
top of an ornamental post or standard. These lamps have 
the mechanism below the arc and are enclosed in a translucent 
diffusing globe. Metallic-electrode lamps are not well adapted 
for general use indoors because of the copious fumes given off. 

56. Standard Sizes. The commercial lamps are also known 
as “ metallic-flame ” or “ luminous-arc ” lamps. The sizes 
made by the varies manufacturers vary slightly in rating. 
Table 6 gives performance data for the lamps made by one 























































































ARC LAMPS 


[CHAP. 3 


30 . 

manufacturer. For 220- and 550-volt, constant-potential 
circuits, 110-volt multiple-series lamps are used. The loss 
of light during the life of one electrode is relatively small 
because of the thorough ventilation of the lamp. 

57. Distribution and Color of Light. The lamp gives the 
maximum candlepower at an angle slightly below the hori¬ 
zontal, and it is therefore well adapted for street lighting where 
most of the light must be directed to points a considerable 
distance from the lamp. The angle for the maximum candle- 
power varies somewhat with different makes of lamps and with 
different sizes, but all types give high candlepower values for 
angles between 10° and 45° below the horizontal. Fig. 10 



Fig. 10.—Light Distribution of Metallic-electrode Arc Lamps. 

Curve A, 4-ampere luminous arc lamp, with refractor and high-efficiency 
electrode. Curve B, 5-ampere luminous arc lamp, with refractor and high- 
efficiency electrode. Curve C, 6.6-ampere luminous arc lamp, with reflector 
and standard electrode. (General Electric Co.) 

shows the distribution of light at various angles above and 
below the horizontal for the pendant type of lamp used for 
street lighting. Two kinds of lamps are made, one having an 
internal reflector, and the other a glass refractor (Fig. 9a). 
Each of these devices is designed to direct most of the light 
at an angle slightly below the horizontal (about 10°), as this 
gives the best distribution for street-lighting purposes. The 
light produced by the metallic-electrode arc is white in color 
and closely resembles daylight. It is therefore very well 
suited for street lighting. 

Mercury-vapor Lamps 

68. Construction. Mercury-vapor lamps are of two kinds: 
the low-pressure, glass-tube type, and the quartz-tube type. 





























PAR. 58] 


MERCURY-VAPOR LAMPS 


31 


The light-producing element in these lamps is an arc of mercury 
vapor confined in a tube of glass or quartz. All of the light is 
produced by the arc and none comes from the electrodes. The 
amount of light given off depends greatly upon the pressure in 
the tube, and this, in turn, is fixed by the temperature. The 
low-pressure type uses a glass tube and operates at a very 
low pressure. The temperature of the arc is therefore low 
(about 200° Cent., or 392° Fahr.). The tube is operated in a 
slightly inclined position. Mercury is used as the lower elec- 



F IG . 11.—Diagram of 110-volt Direct Current Cooper Hewitt Lamp. 

(Low-pressure type). 

1. Starting resistance. 2. Inductance coil. 3. Shifter. 4. Adjuster resist¬ 
ance. 5. Starting band. 

trode, which is connected to the negative terminal of the circuit. 
The other end of the tube is enlarged and contains the positive 
terminal, which is made of iron or graphite. The mercury 
forming the negative terminal is vaporized by the action of 
the current and is condensed in the enlarged part of the tube at 
the top. From this “ condensing chamber ” it runs back to 
the lower terminal, so as to maintain the supply of mercury. 
The tube for a 110-volt lamp is about 50 in. long and 1 in. 
in diameter. When the lamp is not operating, a vacuum exists 
in the tube and there is consequently a high resistance between 
the terminals. The lamp may be started by tilting the tube 


l 
















































32 


ARC LAMPS 


[chap. 3 


until a thin stream of mercury connects the two terminals. 
When the tube is tilted back again this stream is broken, an 
arc is formed and a conducting path of mercury vapor results. 
The tilting may be accomplished either automatically or by 
hand. The arc may also be started by breaking down the 
resistance between the terminals with a spark coil. The mer¬ 
cury arc will not operate unless supplied with direct current. 
For a.c. lamps, it is therefore necessary to provide some method 
of keeping the lower terminal negative. By means of an addi¬ 
tional terminal near the upper end of the tube, the alternating 



Fig. 12. —Diagram of Alternating Current Cooper Hewitt Lamp. 

(Low-pressure type.) 

1. Starting resistance. 2. Inductance coil. 3. Shifter. 4. Starting band. 
5. Series resistance (ballast) to steady arc. 6. Auto transformer. 

current is changed to direct current or “ rectified ” and the 
proper current supply for the arc is maintained. Fig. 11 
shows the arrangement and connections for the 110-volt low- 
pressure, d.c. lamp arranged for automatic starting. The 
tube requires a resistance in series to keep the current at a steady 
value, and also requires inductance coils in series to prevent 
the arc from being extinguished at irregular intervals. The 
“ shifter ” breaks a circuit through the inductance coil, thereby 
applying a high voltage on the “ starting band ” and causing 
the lamp to light. The d.c. lamp is also made to start auto¬ 
matically by tilting, the mechanism used for this purpose being 
similar to that shown in Fig. 13. Fig. 12 shows a diagram of 





















PAR. 58] 


MERCURY-VAPOR LAMPS 


33 


connections of the a.c. lamp. This lamp requires a steadying 
resistance, or ballast, and an inductance coil. A shifter and 
starting resistance to limit the current are also provided. 
Referring to Fig. 12 it will be seen that the line is connected 
across the terminals of an auto-transformer and the negative 
terminal of the lamp is connected to the centre of this trans¬ 
former. The two sides of the 
transformer are connected to 
the two positive terminals of the 
tube. By this means the tube 
is always kept at the proper 
polarity and direct current flows 
through the tube and inductance 
coil. The quartz-tube lamp dif¬ 
fers from the ordinary type of 
lamp in having a very short 
tube, about 4 in. for a 220-volt 
lamp. This results in a very 
much higher pressure and tem¬ 
perature of the arc, with a corre¬ 
sponding difference in the char¬ 
acter of the light produced. 

The quartz lamp operates with 
about atmospheric pressure in 
the tube and the temperature of 
the arc becomes so high that or¬ 
dinary glass would soften. For 
this reason, the tube is made 
from fused quartz. Fig. 13 shows 



Cooper Hewitt Quartz 
Lamp. 


the arrangement and connec¬ 


1. Quartz burner. 2. Magnet 
coil for tilting burner. 3. Armature 
operated by (2). 4. Cutout oper¬ 

ated by (5), open when lamp is 
burning. 5. Inductance coil for 
maintaining the arc. 6. Adjuster 
resistance for setting lamp for 
different supply voltages. 7. 
Starting resistance to limit current 
when arc is formed. 8. Series 
resistance to steady the arc. 


tions for a 220-volt quartz lamp. 

The lamps are arranged for 
automatic starting by tilting 
the burner and require a 
series resistance (8) and an in¬ 
ductance coil (5), the same as the low-pressure lamps. This 
resistance may be adjusted (6) to suit the supply voltage. 
When current is supplied to the lamp, a circuit is formed 
through the magnet coil (2), starting resistance (7), cutout (4) 
and adjuster resistance (6). The magnet then pulls up the 
armature (3) and tilts the tube, thus forming a circuit through 
























































34 


ARC LAMPS 


[CHAP. 3 


the tube, series resistance (8), inductance coil (5) and adjuster 
resistance (6). When current flows in the circuit, the induct¬ 
ance coil opens the cutout (4), which breaks the circuit through 
the magnet coil, restoring the burner to a level position, 
thereby breaking the mercury path in the burner and starting 
an arc. 

69. Applications. Mercury-vapor lamps have been used 
principally for industrial illumination because of the peculiar 
color of the light. They are particularly adapted for lighting 
metal and wood-working plants, textile mills, printing estab¬ 
lishments, warehouses and yards. They are also very well 
adapted for photographic work because the light given off 
is more active photographically than other artificial illuminants. 
The lamps are used extensively for portrait and moving-picture 
studios, and for photo-engraving. Lamps are used on both 
a.c. and d.c., constant-potential circuits. The lamps have 
been found to be especially adapted for illuminating work 
requiring close attention to details. 

60. Standard Sizes. The commercial styles are called Cooper 
Hewitt Lamps. Table 7 gives data on the usual sizes manu¬ 
factured. For indoor use, the low-pressure lamps are provided 
with a spark coil for starting, as explained in paragraph 58. 
For outdoor service and for interior use, where the temperatures 
are likely, to be low, the spark coil type of lamp does not start 
readily, and the tilting type of lamp (either automatic or hand) 
must be used. For d.c. circuits above 110 volts, the tubes are 
burned in series. For a.c. circuits, a small transformer is 
used to step down the voltage to 110. The a.c. lamps may 
be operated on 50 or 60 cycles. They are started in the 
same way as the d.c. lamps. The quartz-tube lamp is at present 
made for d.c., 220-volt, multiple circuits. Where these lamps 
must be operated from a.c. circuits, a specially designed mercury- 
arc rectifier is provided to operate a group of lamps. The power 
consumption of the low-pressure lamp is about 0.48 watt 
per m.h.cp. for both the d.c. and the a.c.' lamps. The 
quartz lamp has a higher efficiency, about 0.30 watt per 
m.h.cp. The life of the tubes is very long, generally several 
thousand hours. The candlepower decreases, however, with 
burning. For the low-pressure type, tests show that the 
candlepower falls to about 80 per cent of the rated value after 
2000 hours burning. When the low-pressure lamp is started, 


PAR. 61] 


MERCURY-VAPOR LAMPS 


35 


the current is momentarily about double the normal value until 
the arc has been established. With the quartz lamp, starting 
cold, the current rush is about four times normal, but this drops 
rapidly as the tube heats up. At the end of three minutes, 
the current is about 50 per cent above the normal value, which 
is reached in from fifteen to twenty minutes. 

61. Distribution and Color of Light. Since the light is given 
off equally in all directions at right angles to the axis of the tube, 
the lamps must always be used with reflectors. These take the 
form of U-shaped metal troughs which enclose the tube on one 
side. Usually these reflectors have a white porcelain surface. 
The reflectors are so designed that all the light is directed below 
the horizontal, the maximum candlepower being vertically 
downward. The light produced by the low-pressure lamps 
has a pronounced bluish-green color, with practically no red 
rays. For this reason, it is not at all adapted for use where 
color values are important or where artistic illumination is 
desired. It is, however, very satisfactory for industrial work. 
The light from the quartz lamp contains more red rays, but it 
is still decidedly greenish in color. The improvement is due 
to the higher temperature of the arc. The quality of the light 
cannot be improved by the use of colored enclosing globes, 
since these could not add any red rays. By using flame-arc 
lamps or tungsten lamps combined with mercury-vapor lamps 
in the proper proportion, the resulting illumination may be 
greatly improved and made more nearly like daylight. The 
low-pressure lamps are sometimes provided with reflectors 
coated with rhodamine enamel, which gives off red rays when the 
lamp is operating. This considerably improves the quality 
of the light produced. These reflectors are called “ light trans¬ 
formers ” or red reflectors.” The quartz lamp when opera¬ 
ting without the enclosing globe is very injurious to the eyes, 
because of certain invisible “ultra-violet” rays.* The ordi¬ 
nary clear glass globe cuts off these rays and renders the light 
safe. 

62. Other Vacuum-tube Lamps. The Moore-tube lamp has 

been used to a limited extent for commercial lighting. It con¬ 
sists of a glass tube varying in length from 60 to 200 ft. and con¬ 
taining a gas at low pressure. A high voltage is applied to 
terminals at each end of the tube, thereby causing the gas 

* See paragraph 65. 


36 


ARC LAMPS 


[CHAP. 3 


to become luminous. The color of the light produced depends 
upon the kind of gas used. With nitrogen gas the light is 
yellow, and with carbon-dioxide gas it closely resembles day¬ 
light. The tubes have a power consumption of about 2.5 watts 
per m.s.cp. when nitrogen is used. This type of vacuum tube 
is no longer used in long lengths for general illumination, but a 
special outfit has been developed to give daylight effects for 
show windows and color matching. The neon-tube has been 
developed in France. This is similar to the Moore-tube, the 
gas used being neon, which is found in very small quantities 
in the atmosphere. Tubes filled with this gas require only 
about one-third the voltage necessary for the Moore-tube, 
and the candlepower per foot of tube is three times as great. 
The light is a golden orange color. Due to the difficulties 
encountered in extracting the neon from the air, the tubes have 
not come into extensive use. In France, they have been made 
in various shapes and sizes, for general illuminating purposes, 
and with the tubes shaped in the form of letters for use in 
advertising. The power consumption is stated to be about 
one-third that for the Moore-tube. 


CHAPTER 4 


PRINCIPLES OF ILLUMINATION 

63. Light. When a body is heated, it finally reaches a tem¬ 
perature where it begins to give off light which is red in color. 
As the temperature is increased the color changes, first to yellow 
and finally to white, and the light efficiency increases rapidly. 
A high temperature is therefore necessary in an incandescent 
lamp. White light is composed of light of three primary 
colors, red, green, and blue, combined in the proper proportion. 
If these proportions are changed the light will no longer be white, 
but will have a color which will depend upon the relative amount 
of each primary color present. 

64. Reflection and Color. Light rays travel in straight lines 
unless interfered with by some medium which absorbs or deflects 



a. Regular refraction. 



Fig. 14.—’Examples of Refraction. 


them from their original course. Refraction occurs when the 
light is deflected slightly from the straight path, in passing 
obliquely through transparent materials like glass or water.. 
Fig. 14 gives examples of refraction. Regular refraction (a) 
occurs when the light passes through wedge-shaped pieces of 
glass such as the ribs of the prismatic type of reflectors. Irreg¬ 
ular refraction ( b ) occurs when the glass surface is rough, as 
in frosted globes. Diffusion (c) occurs when there are par- 

37 





















38 


PRINCIPLES OF ILLUMINATION 


[chap. 4 


tides in the glass which reflect the rays in various directions. 
Globes made of white or translucent glass give this effect. 
Reflection occurs when the light strikes an opaque object and 
is given off again in a different direction. Reflection may 
also occur with transparent objects if the light strikes the 
surface obliquely. Fig. 15 illustrates diffeient kinds of reflec¬ 
tion. Regular reflection (a) occurs when the light strikes highly 
polished surfaces, such as silvered mirrors and polished metal 
or wood-work. With irregular reflection (6) the reflected light 
rays do not all leave the surface at the same angle, but are 
directed at different angles. This type of reflection occurs 
with aluminum-finished reflectors and etched-glass surfaces. 
Subsurface reflection (c) occurs when the light passes through 




oVo o N 6 O 

o o o O O 


a. Regular reflection. b. Irregular reflection, c. Sub-surface reflection. 


Fig. 15. —Examples of Reflection. 


a transparent portion of a surface and is reflected at various 
angles by opaque particles located beneath the surface. This 
type of reflection occurs with porcelain enameled and painted 
surfaces. 

A portion of the light which strikes an opaque object is always 
absorbed, the amount depending upon the character of the 
object. Substances of different kinds do not reflect light in 
the same way, and from this result differences in appearance 
and in color. A black body reflects no light, and hence it can 
be seen only by contrast with nearb}' objects which reflect light. 
A body illuminated by white light will appear white provided 
it reflects the primary colors of which white light is composed, 
in such an amount as to maintain the proper proportion of each. 
On the other hand, if it reflects more light of one color than 
another, the object will not appear white even when illuminated 
by white light. A body which appears red reflects the red 
light and absorbs the other colors. It should be noted, how¬ 
ever, that if an object appears of a certain color, the light which 
is used to illuminate the object must also contain that color. 
For this reason, the mercury-vapor lamp, which produces 







PAR. 65 ] 


REFLECTION AND COLOR 


39 


no red rays, gives very peculiar effects. Objects which would 
appear red, when illuminated by white light, appear black 
under the mercury-vapor lamp, since no light rays are reflected. 
Any color which is composed partly of red would not appear in 
its true color, and objects normally white have a greenish-blue 
color when illuminated by the mercury-vapor lamp. The color 
of the light in a room is not always the same as the color of the 
source. If white light is produced in a room with green walls, 
most of the light which strikes the walls is absorbed, while the 
green rays are reflected, thus giving a green tint to the illumina¬ 
tion. Dark walls and ceilings result in a low efficiency of the 
lighting system, since much of the light produced is absorbed 
and lost. 

65. Quality of Light. The light produced by an artificial 
source is usually compared with daylight, which is commonly 
called white light. Artificial illuminants in general do not 
exactly reproduce daylight effects, but by means of special 
screens some of the more efficient lamps can be made to closely 
approximate white light. Incandescent lamps are in general 
somewhat yellow, while some arc lamps give a bluish light. 
Light vibrations, at rates slightly higher than those which 
are visible, produce “ ultra-violet rays,” more correctly called 
ultra-violet radiation. These are produced by most light 
sources, but usually in so small an amount as not to be inju¬ 
rious to the eyes. The electric arc and the quartz-tube lamp, 
however, produce a large amount of these radiations and are 
therefore dangerous to the eyes, unless shielded by ordinary 
glass, which largely cuts off the ultra-violet rays while still 
allowing the visible rays to pass freely. 

66. Units.* The candlepower or intensity of light emitted 
by a lamp in a given direction is expressed in terms of a stand¬ 
ard source of light. Formerly this was a special form of candle, 
burning under specified conditions. The present unit, the 
international candle, is however based upon certain standard 
incandescent lamps maintained by the Bureau of Standards at 
Washington. Thus a lamp which gives, in a specified direc¬ 
tion, a light intensity twenty times as great as the standard 

* The term “unit” as used in this paragraph refers to its usual meaning 
of a standard of measurement. The term “light-unit” sometimes abbre¬ 
viated unit, is commonly used in lighting work and will be explained in a 
later paragraph, j 


40 


PRINCIPLES OF ILLUMINATION 


[chap. 4 


candle would be said to give 20 cp. in that direction. Arti¬ 
ficial illuminants do not, however, give the same candlepower 
in all directions; thus a 100-watt tungsten lamp which may- 
give 96 cp. in a horizontal direction will give only about 30 cp. 
vertically downward (see Fig. 20). It is unfair, therefore, to 
compare different kinds of lamps upon the basis of the candle- 
power in one direction only. A more accurate comparison 
may be made when the average candlepower is employed. 
This average may be taken in several ways. We may, for 
example, measure the candlepower of an incandescent lamp in 
a large number of directions all of which are in a horizontal plane 



Fig. 16. —Measurement of Mean Horizontal Candlepower. 


through the center of the lamp. Fig. 16 illustrates the arrange¬ 
ment, the radial lines equally spaced representing the various 
directions in which the candlepower is to be measured. The 
average of the candlepower values so obtained gives the mean 
horizontal candlepower (abbreviated m.h.cp.) of the lamp. 
In actual practice, this value would be obtained, not by making 
a large number of readings, but by rotating the lamp, thereby 
measuring directly the average candlepower. Until recently, 
incandescent lamps were rated commercially on the basis of 
their mean horizontal candlepower. It is sometimes desired to 
express the candlepower of a lamp in other directions than the 
horizontal. We can imagine the lamp located at the centre of 
a globe or sphere. If we should measure the candlepower of 








PAR. 66] 


UNITS 


41 


the lamp at a large number of points equally spaced on the 
inside surface of this sphere, we would find a wide variation in 
the candlepower of the lamp. If we average all of these read¬ 
ings we obtain the mean spherical candlepower (abbreviated 
m.s.cp.) of the lamp. In this way, we take account of all 
the light produced, and comparisons between different lamps 
can therefore be made more accurately than by other methods. 
For this reason, commercial ratings of lamps are now commonly 
expressed in terms of the mean spherical candlepower. If we 
consider only the lower half of the sphere, the averaged readings 
would give the mean lower hemispherical candlepower (abbrevi¬ 
ated m.l.h. cp.). This term has been used commonly in the 
commercial rating of arc lamps. The candlepower of a lamp is 
a measure of the brightness or intensity of the light in a given 
direction, and this candlepower would remain the'same irrespec¬ 
tive of the distance between the lamp and the point of measure¬ 
ment. Thus, if a tungsten lamp was found to give 100 cp. 
when measured in a certain direction at a distance of 10 ft., 
another measurement made at a distance of 100 ft. in the same 
direction would be found to give the same candle-power. This 
would hold true for all distances in a given direction, except 
when the distance becomes so great that some of the light is 
absorbed by the atmosphere. The illumination , however, de¬ 
creases rapidly with increase in distance, as will be explained 
later. The output of light from a lamp is called the light 
flux and is measured in lumens. We can compare a lamp 
with a metal ball supplied with water under pressure. If a 
large number of small holes, equally spaced, are drilled in the 
ball, then neglecting the effect of the supply pipe, jets of 
water would issue from the ball in all directions. These jets 
may be compared with the light flux given off by a lamp. The 
total amount of water issuing from the ball would be the aggre¬ 
gate from all the holes and would correspond to the total 
light which would be given off by the lamp. This would be 
expressed in lumens. We may speak of the total lumens 
or total light flux produced by a lamp which includes the light 
given off in all directions, or we may consider only a particular 
portion of the light given off, as for example, the lumens in the 
lower hemisphere. We may also use the term lumens per watt, 
which is found by dividing the total lumens output by the watts 
required to produce this output. This quantity is convenient 


42 


• PRINCIPLES OF ILLUMINATION 


[chap. 4 


for comparing the efficiency of two lamps. For example, a 
500-watt tungsten lamp gives 8050 lumens, or 16.1 lumens 
per watt. A certain arc lamp requires 495 watts and has an 
output of 3650 lumens, or 7.37 lumens per watt. We can 
therefore say that the tungsten lamp is twice as efficient as the 
particular arc lamp considered. In calculating values of 
lumens per watt, it is customary to use the total watts supplied 
to the lamp so as to include the losses in the lamp mechanism. 
Referring to the two lamps just compared, it should be remem¬ 
bered that the candlepower of the tungsten lamp in certain 
directions would not be twice as great as the arc lamp. In 
fact the candlepower of the arc lamp at an angle of 30° below 
the horizontal is 530, while the tungsten lamp at the same 
angle gives about 700 cp. The candlepower may be changed 
by the use of reflectors,* but the total light output in lumens 
is the same whether a reflector is used or not, provided the 
power consumed by the lamp remains the same. The lumens 
given off in a definite direction , for example downward, 
may be changed materially if a reflector is used; in other 
words, the flux, which originally was directed upward, might now 
all be directed downward, except for the amount absorbed by 
the reflector. In fact, a reflector may be thought of as a sort 
of nozzle which serves to turn the light into a useful direction. 
Fig. 17 illustrates the effect of reflectors upon the light dis¬ 
tribution. For purposes of comparison, it is also necessary to 
determine the amount of illumination produced at a given point 
by the light source. This is expressed by the term foot-candles t 
or lumens per square foot. Thus, if the flux of light on a given 
surface is such that there are 4 lumens per square foot, then we 
would say that the illumination produced was 4 foot-candles. 
It is necessary to distinguish carefully between the candle- 
power of a lamp and the quantity of light which reaches a 
particular object. The- candlepower of a lamp in a certain 

* See paragraph 79. 

t By definition, a foot-candle is the illumination produced on a surface 
situated 1 ft. distant from a light source of 1 cp. The foot-candle illumi¬ 
nation of a surface can be obtained in any case by dividing the candle 
power of the lamp in the direction considered by the square of the distance 
in feet from the lamp to the given surface. One foot-candle is about equal 
to the illumination produced on a surface situated 5 ft. horizontally from 
a 25-watt tungsten lamp without reflector. This unit is now .being replaced 
by the term lumens per square foot. 


PAR. 66] UNITS 



43 


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■4J o 
.22 o 


1—1 fcfl 

a .2 

° 2 

a, * 




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t. 

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44 


PRINCIPLES OF ILLUMINATION 


[CHAP. 4 


direction is definite and would be the same if measured close 
to the lamp or at a distance of hundreds of feet, as has already 
been explained. The quantity of light which falls on a given 
area, however, that is, the lumens per square foot, depends 
upon the distance from the lamp. Suppose a surface (a, Fig. 
18) is placed at right angles with the light from a lamp and 
distant 5 ft. from it; the light, which we will consider con- 



Fig. 18. —Variation of Illumination with Distance. 


centrated at a point, would illuminate the surface practically 
uniformly. Suppose this illumination amounts to 16 lumens 
per square foot of surface, in other words 16 foot-candles. 
If we now cut an opening of 1 sq.ft, area in this surface, we shall 
allow 16 lumens of light flux to pass through and'•strike the 
surface ( b ), which is at twice the distance from the light. Since 
the light travels in straight lines, it will spread over more than 
1 sq.ft, of the surface of ( b ). In fact, it will be seen that the 
surface illuminated will measure 2 ft. on a side, giving a total 
area of 4 sq.ft. Hence the light flux, amounting to 16 lumens 



























PAR. 67] 


DISTRIBUTING CURVES 


45 


is now spread over four times the surface and the illumina¬ 
tion will be one-fourth as great, or 4 lumens per square foot 
(4 foot-candles). This law is expressed by saying that the 
illumination of a surface varies inversely as the square of the 
distance from the light source. While this inverse square 
law is exactly true only when the light is concentrated at a 
point and the surfaces are spherical, it is very closely correct 
for ordinary lamps and flat surfaces at considerable distances 
from the light. There are many cases in actual practice, how¬ 
ever, where the law should be used with care because of the 
changes occasioned by the use of reflectors.* If the light 
does not strike the surface at right angles, the illumination for 
a given amount of light is less, since it is spread over a 
larger area. The total lumens produced by a lamp can 
be found by multiplying the mean spherical candlepower by 
12.57. 

67. Distribution Curves. The candlepower of a lamp in 
various directions is shown by distribution curves as in Fig. 
20. The lamp is at the centre of a number of concentric cir¬ 
cles equally spaced and marked with candlepower values, using 
a suitable scale. Equally spaced radial lines are drawn from 
the lamp as a centre, representing the different directions in 
which the readings are taken. These, of course, are all in the 
same plane, but this plane may be taken either vertically or 
horizontally through the centre of the lamp. For most purposes, 
however, a vertical plane is most convenient, since nearly all 
modern lamps give a uniform candlepower at all points in the 
horizontal plane. The curves shown in Fig. 20 are taken 
in a vertical plane with the lamp and reflectors pointing down¬ 
ward in the usual way. If we select any of the radial lines 
(or imagine one if there is none in the particular direction 
chosen) this line, by its angle with the horizontal or vertical, 
indicates the direction in which the candlepower is given, 
and by its length to the point where it meets the curve, the 
value of the candlepower in this direction. For example, 
referring to curve (A) Fig. 20, the candlepower in a horizon¬ 
tal line is 54 and in a vertical line directly below the lamp 
it is 90 cp. At 15° below the horizontal, the candlepower 
is 60 and so on. The other curves may be read in the same 
way. 

* See paragraphs 79 and 119. 


46 


PRINCIPLES OF ILLUMINATION 


[chap. 4 


68. Systems of Illumination. Objects may be illuminated 

either by the direct or by the indirect method and the systems 
of illumination are classified according to the method used. 
With the direct system, the largest proportion of the light is 
directed on the surface to be illuminated without being reflected 
by walls or ceiling. With the indirect system, all of the light 
is projected to the ceiling or walls and is then reflected on the 
object to be illuminated. The semi-indirect system is a com¬ 
bination of the other two, in which the greater proportion of 
the light is reflected from the ceiling or walls, the remainder 
reaching the object directly through diffusing globes which form 




a. Direct. b. Semi-indirect. c. Indirect. 

Fiq. 19.—Systems of Illumination. 

a part of the lighting unit. In the direct system, a considerable 
part of the total light may be reflected from the ceiling and 
walls if they are light in color and if transparent reflectors 
are used on the lamps. Fig. 19 illustrates these different 
systems. 

69. Requirements for Artificial Illumination. The plan¬ 
ning of a satisfactory system of illumination requires consider¬ 
ation of (1) intensity of illumination, (2) steadiness of the 
light, (3) uniformity of illumination, (4) diffusion of illumina¬ 
tion, (5) elimination of glare, (6) color-value of the light, (7) 
appearance, (8) efficiency. These are not given in the order 
of their importance, in fact it would be difficult to do so, as the 
relative importance of these factors varies with the kind of 
installation considered. 
















PAR. 70] 


INTENSITY OF ILLUMINATION 


47 


70. Intensity of Illumination. Since the purpose of all illu¬ 
mination is to enable objects to be seen, the first consideration 
is to provide sufficient illuminating intensity. Fortunately the 
eye can adapt itself to a wide range of intensities by changes 
in the size of the pupil of the eye, and adjustments of its internal 
parts. Changes in the size of the pupil occur rapidly and tend 
to adjust automatically the amount of light which enters the 
eye. A further adaptation is made internally to suit a varia¬ 
tion in the amount of light admitted. In brief, the eye attempts 
to adjust itself to secure comfortable vision. If the intensity 
is too low, the object cannot be seen distinctly because not 
enough light enters the eye even when the pupil is enlarged as 
much as possible; on the other hand, if the intensity is too high, 
more light is admitted to the eye, even when the pupil is con¬ 
tracted, than can be accommodated, and this results in dis> 
comfort and sometimes permanent injury to the eye. We 
can see well and without discomfort with daylight intensities 
ranging from 1 to 500 foot-candles, while under artificial 
lighting conditions the eye will adjust itself to intensities from 
less than 1 foot-candle to several hundred foot-candles. A 
smaller range of intensities for artificial illumination will, 
however, meet the usual requirements. The best intensity 
depends upon the use which is made of the illumination. For 
reading, 3 to 4 foot-candles intensity gives satisfactory results, 
while for work requiring close application to detail, such as 
drafting or tool-making, high intensities of from 5 to 10 foot- 
candles are necessary. From 1.5 to 3.0 foot-candles is suf¬ 
ficient for warehouses, piers, etc. In street lighting, the average 
intensity is usually about 0.05 foot-candle. The values of 
illumination intensity required for proper vision are influenced 
greatly by the surroundings and the contrasts in intensity which 
exist. The methods of choosing a suitable intensity are dis¬ 
cussed in Chapter 7. 

71. Steadiness of the Light. If the intensity of illumina¬ 
tion fluctuates, the pupil of the eye contracts and expands to 
adjust the eye to these changes. If the fluctuation is rapid 
and continuous, the muscles of the eye are fatigued and eye- 
strain results. Fluctuations such as occur with arc-lamps, 
while unpleasant, do not as a rule cause fatigue. If the 
fluctuations are rapid enough, the eye does not follow the changes, 
but adjusts itself to the average intensity. An example of this 


48 


PRINCIPLES OF ILLUMINATION 


[CHAP. 4 


character of fluctuation is the incandescent lamp operated 
by alternating current. At 60 cycles, no fluctuation can be 
detected by the eye, although there is variation in the candle- 
power of the light. If the frequency is decreased sufficiently, 
the eye is able to distinguish the variations in candlepower 
and we say the light flickers. The lowest frequency limit 
suitable for all purposes is about 35 cycles, although 25 cycles 
has been used satisfactorily for general illumination except 
where very small size tungsten lamps are used. Gas-filled 
lamps of a given ampere rating show less flicker than the 
vacuum-type lamps of the same rating. Lamps with metal 
filaments are less sensitive to fluctuation in voltage than carbon 
lamps of the same size of filament. The effect is more notice¬ 
able for a thin filament of a given substance than for one which 
is thick. The fluctuation in any case is less noticeable for low 
intensities of illumination. 

72. Uniformity of Illumination. Until within a few years 
no attempt was made to secure even approximately uniform 
illumination for industrial or commercial lighting. Individual 
lamps, usually of 16 cp. size, were provided for each worker 
or machine and little or no attempt was made to provide 
general illumination of the room. Local lighting of this kind 
leads to deep shadows and excessive contrasts and is frequently 
a cause of eye fatigue and a fruitful source of accidents. It is 
now the custom to provide a fairly uniform general illumina¬ 
tion, which may be supplemented by localized illumination 
when the nature of the work makes this desirable. If the 
foot-candles intensity is the same over the entire working sur¬ 
face or plane* the illumination is said to be uniform. It is 
never necessary to secure exact uniformity, since the eye cannot 
detect small variations. 

73. Diffusion of Illumination. This should be distinguished 
from diffusion of the light from the lamp, which is considered 
in the next paragraph. If the light which illuminates an object 
comes from a number of different directions the illumination 
is said to be diffused. If all the light comes from a single 
source, for example a tungsten lamp, then the diffusion is poor 
and there are likely to be deep shadows and extreme contrasts 
in the illumination of different parts of the objects in the room. 
If the light comes from a number of sources, or if it is reflected 

* This is usually assumed to be a horizontal surface 30 in. above the floor. 


PAR. 74] 


GLARE 


49 


first to the ceiling of the room, thus increasing the light-giving 
area, better diffusion is secured, shadows are eliminated to a 
greater or less degree and the glare from polished surfaces is 
avoided. The highest degree of diffusion is secured with indirect 
or semi-indirect systems. With a direct system, satisfactory 
diffusion for most purposes can be secured if the lamps are closely 
spaced. 

74. Glare. Glare is caused by excessive brightness of sur¬ 
faces or objects in the field of vision. The worst examples of 
glare occur when the light source itself is in view or an image 
of the light source is reflected from some polished surface. 
When a bright surface is in the field of vision, the eye tries to 
adjust itself to the intensity of the bright surface; consequently 
an insufficient amount of light enters the eye from other objects 
and they are therefore seen only indistinctly, if at all. It 
is impossible, for example, to distinguish distant objects with a 
bright light nearby in the line of vision. Glare, besides 
reducing a person’s visual power, may result also in eye-strain 
or, if long continued, may even permanently injure the eyes. 
All forms of modern illuminants, except the vapor-tube lamps, 
are so bright that the bare lamp should never be placed in the 
usual line of vision. If it is necessary to place the units where 
they are visible when looking in the usual directions, enclosing 
globes of opaque glass or frosted lamps should be used. With 
incandescent lamps, the reflectors usually enclose the lamps 
sufficiently to protect the eyes. Glare due to light reflected 
from a glazed paper, polished table top or even polished metal 
parts in process of manufacture may often be nearly as objec¬ 
tionable as the glare from an exposed lamp. Such surfaces 
therefore are to be avoided where possible, and in other cases 
diffusion of the light and a suitable direction for the light 
will eliminate the glare. 

76. Color Value of Light. The color value of the light is fre¬ 
quently very important. Thus the nearly white light of the 
metallic-electrode arc lamp is particularly desirable for street 
lighting and is superior for this purpose to either the tungsten 
lamp or the flame-arc. On the other hand, for smoky or dusty 
places the yellow-flame arc seems to be particularly effective. 
For interior lighting, in general, the light of the tungsten 
lamp, which is slightly yellow, is particularly pleasing. In 
some cases daylight effects are required for interior lighting, 


50 


PRINCIPLES OF ILLUMINATION 


[chap. 4 


necessitating the use of special blue-glass tungsten units, Moore- 
tubes or other devices. Some lamps, for example the mercury- 
vapor arc, give a light so different from daylight that they 
could not be used where the color of light is important, as in 
stores, offices, etc. 

76. Appearance. The appearance of a lighting installation 
is affected principally by the kind and shape of the reflector 
and the proportions of the fixtures. No attempt can be made 
here to cover in detail the requirements for securing a pleasing 
appearance, more than to outline a few fundamental rules which 
would apply particularly to commercial and industrial lighting 
installations. It would be obviously bad taste to use metal 
reflectors for lighting residences, stores, offices and similar 
places. On the other hand, glass reflectors are not in general 
suited for lighting factories. 

77. Efficiency. In most commercial and industrial lighting 
installations, efficiency must be carefully considered. This 
does not mean, however, that we should use the cheapest reflec¬ 
tor which can be purchased, nor does it mean that Gem lamps 
should be used because they cost less than tungsten. The 
installation which costs the least to install is generally the 
most expensive to operate, and usually has to be modified 
later at considerable additional expense. The time has passed 
when superintendents and managers are satisfied with a light¬ 
ing installation which will be just sufficient to allow the opera¬ 
tives to continue their work. At present it is realized that 
a lighting installation which is designed with regard to proper 
intensity, shielding of the lamps and the other factors already 
mentioned, not only results in better satisfied workmen but also 
shows definite results in a material increase in quantity and an 
improvement in the quality of product turned out during the 
lighting hours. It also has a very important effect upon the 
number of accidents. The use of low-efficiency lamps such 
as the Gem or the ordinary carbon filament lamp for general 
illuminating purposes is never justified, as it can be easily shown 
that the saving in power effected by using high-efficiency 
lamps soon makes up the difference in the initial cost of the 
lamps. True efficiency therefore requires that proper reflec¬ 
tors and high-efficiency lamps be used and that the spacing 
and arrangement of the units shall be such as to fulfill the 
requirements given in the previous paragraphs. 


CHAPTER 5 


LIGHTING ACCESSORIES 

78. Purposes of Lighting Accessories. The accessories used 
with artificial illuminants include reflectors, which are used 
primarily to redirect the light in a useful direction; diffusing 
globes and bowls, which serve principally to conceal the lamp 
and reduce the glare; and shades, which are used to improve 
the appearance of the unit. 

79. Purpose of Reflectors. In order to use artificial illu¬ 
minants effectively, the light produced must be directed in 
proper proportions in certain definite directions. To secure 
high efficiency, most of the light must be used to illuminate 
the working area, but frequently, in order to improve the ap¬ 
pearance of the room and to prevent eye-strain, it is also neces¬ 
sary to illuminate the walls and ceiling. The proportion of 
light used for each purpose depends upon the system of illumi¬ 
nation employed. In most cases, the light from a bare lamp, 
without reflector or other accessory, is not distributed properly. 
The first purpose of a reflector, therefore, is to redirect the light 
in the desired direction. Properly designed reflectors also 
enclose the lamp to a considerable extent and thus shield it 
from direct view. Glass reflectors allow a certain amount of 
light to pass through and thus give a more general distribu¬ 
tion of the light. All reflectors, whether of clear or translucent 
glass or of metal, absorb some of the light which strikes them. 
Well-designed reflectors, however, do not absorb as much 
light as ordinary walls or ceilings, and since a large part of the 
total light produced by a lamp must be reflected in some way 
before it can become useful, it is necessary, for high efficiency, 
to provide reflecting surfaces which absorb a minimum amount 
of light. The effect of different types of reflectors upon the 
distribution of light from a lamp is illustrated in Fig. 17. The 
figures for the amount of light in different directions are repre¬ 
sentative of high efficiency reflectors of various types used for 

51 


52 


LIGHTING ACCESSORIES 


[chap. 5 


direct lighting systems. The effect of reflectors upon the candle- 
power of a lamp in a given direction is shown in Fig. 20. These 
curves are for different styles of prismatic-glass reflectors for 
direct lighting. It will be seen that the bare lamp without 
reflector gives 30 cp. directly below the lamp and 70 cp. at ^0° 
from the vertical. As the light given off at angles greater than 



Fig. 20.—Light Distribution from Tungsten Lamp with Prismatic 

Glass Reflectors. 

100-watt Mazda lamp operating at 1.02 watts per m.h. candle. A. Extensive 
reflector. B. Intensive reflector. C. Focussing reflector. D. Bare lamp. 


60° cannot usually be employed very effectively, reflectors must 
be used to redirect the light. With reflector A, the vertical 
candlepower is increased to 90, while with reflector C, the 
candlepower is 400. The candlepower in the 60° line is of course 
decreased. If we consider the total light flux in the zone from 
0° to 60°, we find that the lamp alone gives 159 lumens, while 
with reflectors A, B , and C, it is respectively 389, 433, and 469 


































































PAR. 80 ] REFLECTORS FOR DIRECT LIGHTING SYSTEMS 53 


lumens. The total lumens given off by the lamp are the same 
with or without the reflector, but the latter serves to direct 
a larger proportion of the total light flux in a useful direction. 
The bowls used for indirect and semi-indirect systems are also 
to be classed as reflectors. Reflecting surfaces are made of 
polished metal, silvered glass, prismatic glass, opal glass, 
enamelled metal and aluminum paint. Reflectors are used 
for both incandescent and arc lamps. 

80. Purpose of Diffusing Globes. Globes or balls that 
entirely surround the light source are intended primarily to 
reduce the brilliancy of the light. In doing this, they also 

V 

affect the distribution of light, the amount of this change 
depending upon the design of the unit. In some cases globes 
are also used to modify the color of the light. Globes may be 
made of prismatic glass; ground-glass, produced by acid 
etching or sand-blasting; and translucent or white glass of 
various kinds such as opal, alabaster, milk, etc. A considerable 
amount of light is absorbed by glass globes. For ground- 
glass, the absorption is about 10 per cent. Translucent glass 
absorbs from 15 to 30 per cent. This results in lower efficiency 
for the lighting installation. Diffusing globes therefore are 
chiefly employed where decorative effects are desired at the 
expense of efficiency; or where, by the use of gas-filled tungsten 
lamps, a reasonably high efficiency can be secured in spite of 
the light absorbed. 

81. Shades. Shades are intended primarily for decorative 
effect, but they generally enclose the lamp to a considerable 
extent and thus shield it from view. Glass shades are made 
in a large variety of shapes and kinds of glass, and can be 
used to produce very artistic effects. All shades are very 
inefficient in their distribution of light. Metal shades are to 
be avoided, as reflectors which properly distribute the light as 
well as shade the lamp are preferable. 

Accessories for Direct Lighting Systems * 

82. Classes of Reflectors. Reflectors may be classified 
according to the manner in which they redistribute the light 
as follows: (1) extensive; (2) intensive; (3) focussing; (4) 
distributing; (5) angle. The characteristics of classes 1, 2, 

* The following discussion applies more particularly to incandescent lighting. 


54 


LIGHTING ACCESSORIES 


[CHAP. 5 


and 3 are shown in Fig. 20. It will be seen that (a) gives the 
widest distribution of the light, (c) concentrates the light, 
and ( b ) gives a distribution intermediate between the other two. 
The extensive type (a) would generally be used for low ceilings, 
the intensive ( b ) for medium ceilings and the focussing (c) 



a-DOME TYPE 



Style of reflector. Characteristic ligdit distribution. 

Fig. 21. —Typical Steel Reflectors for Tungsten Lamps. 

for high ceilings. The distributing type gives a wider distribu¬ 
tion than the extensive type and is used for large areas and for 
street lighting. Fig. 21a shows this type of reflector'and gives 
the distribution curve. The angle reflector throws more of 
the light to one side, as shown in Fig. 21c. This type is used 
for lighting benches or particular parts of the work or machine, 
and for show windows. 
































PAR. 83] REFLECTORS FOR DIRECT LIGHTING SYSTEMS 55 

83. Construction of Reflectors. Reflectors are made of metal, 
prismatic glass and white glass. Metal reflectors are usually 
made of steel, but aluminum and brass are also used to some 
extent. The reflecting surface on metal reflectors is usually 
white porcelain enamel, white paint or aluminum paint. En¬ 
amelled steel reflectors are the most satisfactory type of metal 
reflector for general use, as the surface has a high reflecting 
power, is permanent, easily cleaned, will resist acid fumes and 
heat, and will withstand exposure to weather in outdoor service. 
Reflectors having white painted surfaces are cheaper than the 
enamel type, but the surface rapidly deteriorates and they are 
therefore less efficient. The aluminum finish reflectors, when 
new, have as good reflecting power as the enamel type, and 
where the interior of the reflector is in the usual direction of 
vision, this style is preferable, as it diffuses the light and gives 
less glare. The aluminum surface, however, docs not main¬ 
tain its high efficiency during use, and the surface is difficult 
to clean. Steel reflectors with the aluminum finish are some¬ 
what cheaper than the enamelled type. Metal reflectors are 
usually of the dome type (Fig. 21a), the bowl type (6) or the 
angle type (c). The bowl type encloses the lamp more than the 
dome type, but in most cases it is possible to obtain from 
10 to 15 per cent more useful light when the dome type is used. 
Where it is possible, therefore, to mount the lamp high enough 
to be out of the normal direction of vision, the dome type 
should be used. This type of reflector gives a distributing form 
of light distribution (Fig. 21a), while the bowl type may be 
obtained in the extensive, intensive, or focussing types, giving 
distributions similar to those illustrated in Fig. 20, except that 
no light is directed above the horizontal. Prismatic glass 
reflectors are made from clear glass, with the reflecting surface 
composed of deep ribs on the outside. These ribs, to give 
high reflecting efficiency, must be carefully proportioned. 
In this type of reflector (Fig. 20 and Fig. 22 A) a portion of 
the light striking the reflector passes through it and is directed 
against the ceiling and walls. The remainder of the light is 
reflected from the inside surface and is directed downward in 
a predetermined manner. In some cases, the inside surface 
of prismatic glass reflectors is given a velvet finish by etching 
the glass. This diffuses the light and reduces the glare. Pris¬ 
matic-glass reflectors are made to give extensive, intensive or 


56 


LIGHTING ACCESSORIES 


[CHAP. 5 



Fig. 22.—Typical Reflectors of Modern Design. 









PAR. 84] 


HOLDERS FOR REFLECTORS 


57 


focussing distribution of light, the style chosen depending 
upon the height and spacing of the lamps, as explained in 
Chapter 7. they can also be obtained in the angle type. 
White-glass reflectors have smooth surfaces except where 
they are fluted to improve their appearance. They are made of 
many kinds of glass and in a wide variety of shapes. The forms 
generally used are the deep bowl (Fig. 22 g) and the shallow 
bowl. In general, they are slightly less efficient than the 
prismatic-glass reflectors and do not control the distribution 
of light as accurately. The superior appearance of the white- 
glass reflectors results in their use for interior lighting where a 
specially pleasing and artistic installation is required, as in 
stores, offices, etc. These reflectors are made to give either 
a distributing or an extensive distribution. With the direct 
system, using white-glass reflectors, from 35 to 70 per cent of 
the light produced by the lamps reaches the working area, 
the value depending upon the color of the walls and ceiling. 
With metal reflectors, the useful light varies from 30 to 75 per 
cent. Opaque-glass reflectors are made with a silvered 
reflecting surface which is rippled or waved to partly diffuse 
the light and prevent streaked lighting effects. These reflec¬ 
tors are more efficient than either the translucent glass or metal 
reflectors, but like the latter, do not illuminate the ceiling of 
the room. They are also rather fragile when made in large 
sizes. For these reasons they find their chief application in 
window and show-case lighting’. They are also used very 
effectively for indirect lighting units. 

84. Size of Reflector. The size of a reflector varies with the 
candlepower of the lamp. A particular* size of either a metal 
or glass reflector is suited for only one or two sizes of lamps. 
If other sizes are used, the lamp may be exposed too much and 
the distribution of the light will be greatly changed. 

85. Holders for Reflectors. In order that the light may 
be properly distributed, the lamp must be correctly located 
in the reflector. If the lamp is placed too high or too low, 
the distribution of light will be altered and the desired re¬ 
sult will not be obtained.* Care in the use of the proper 
holder is especially necessary with the prismatic glass reflec¬ 
tors, which are carefully designed to give a definite light distri¬ 
bution. The manufacturer’s recommendations with regard to 

* See paragraph 128. 


58 


LIGHTING ACCESSORIES 


[CHAP. 5 


holders should therefore be carefully followed. Fig. 23 (a) 
and (6) shows holders used principally for glass reflectors. 
Style (a) is used for lamps up to 100 watts and (6) for larger 
lamps having a skirted base. These holders attach directly 
to the shell of a standard, medium-base socket. On mogul- 
base sockets, the holder is generally attached permanently 
to the socket shell. With metal reflectors a form of holder 
shown in (c) is used. The notched top of the reflector is gripped 
by three prongs which are attached to the socket and are 
held together by a steel ring. This construction is used for 
both medium- and mogul-base sockets. For medium-base 
lamps a socket which is combined with the reflector in one piece 



Fig. 23. —Holders for Reflectors. 

a. Holder for glass reflectors with regular-base lamps, b. Holder for glass 
reflectors with skirted-base lamps, c. Holder for metal reflectors. 


is frequently used. This is particularly adapted for industrial 
establishments and for outdoor service. Fig. 21 (a) and (c) 
shows this type of reflector.. Fig. 21(6) shows the type which 
attaches to an ordinary lamp socket. 

86. Heating Effect. As has been previously stated, most 
of the energy supplied to a tungsten lamp is given off in the 
form of heat. Thus with large tungsten lamps a very high 
temperature of reflector and socket is likely to result unless 
the unit is properly ventilated. The gas-filled tungsten units 
particularly need ventilation, not only because the units are 
usually of large size, but also because the heat is carried 
into the neck of the lamp by the circulation of the gas in 
the bulb. With glass reflectors, provision is made for venti- 



































PAR. 87] 


APPLICATIONS OF REFLECTORS 


59 


lation by openings in the holders (see Fig. 23). If enclosing 
balls or hemispheres are used, openings must be provided 
at the top for ventilation, and sometimes a small hole is pro¬ 
vided at the bottom of the ball. The one-piece metal reflectors 
(Fig. 21 (a) and (c)), which are closed at the top, are particularly 
likely to reach high temperatures when used with large gas- 
filled tungsten lamps. These reflectors are therefore equipped 
with porcelain receptacles designed to withstand a high tem¬ 
perature and are wired with asbestos-covered wire. It is usual 
also to provide ventilating openings in the neck of the reflec¬ 
tor, where the receptacle is located. This arrangement is 
shown in Fig. 21 (a) and (c). Metal reflectors arranged to 
attach to ordinary sockets by means of shade-holders allow 
better ventilation, but cannot be used where they would be 
exposed to severe weather conditions. The so-called weather¬ 
proof sockets, made of a moulded composition which softens 
when heated, should not be used with large-size tungsten lamps. 

87. Effect of Dust. Dust on reflectors reduces their reflecting 
efficiency. Tests show that after six months’ use, under usual 
industrial conditions, tungsten lamps equipped with the reflec¬ 
tors indicated show a reduction in illumination of the working 


surface as follows: * 

A—Dome-enameled steel reflector. 15% 

B—Bowl-enameled steel reflector. 18 

C—Dense opal glass reflector. 20 

D—Prismatic glass reflector. 25 

E—Light density opal glass reflector. 32 


It is apparent that reflectors and lamps should be cleaned period¬ 
ically if a reasonably high efficiency is to be maintained. 

88. Applications. Porcelain-enameled steel reflectors are 
the best type for general use in industrial lighting because of 
their high reflecting efficiency, their durability and the ease with 
which they may be kept clean. In localities where there is 
little dust, the prismatic or white-glass reflectors may be used 
to advantage, particularly if the units are mounted where they 
are not likely to be struck. Metal reflectors allow practically 
no light to reach the ceiling, but this is not objectionable in 
most cases. Glass reflectors, which allow a portion of the light 
to pass through and illuminate the ceiling, give a more pleasing 

* From Bulletin No. 20, National Lamp Works of the General Electric Co. 







60 


LIGHTING ACCESSORIES 


[chap. 5 


effect. To obtain high efficiency from glass reflectors, however, 
the ceiling and walls must be light in color; therefore metal 
reflectors would be preferable where walls and ceiling are dark 
or where the lamps are mounted a considerable distance below 
the ceiling. Metal reflectors are best adapted for lighting 
steel mills, foundries, metal- and wood-working establishments, 
cement mills, tanneries, warehouses and power houses. Glass 
reflectors are better adapted for use in offices, drafting rooms, 
bakeries, laundries, printing establishments, clothing-manufac¬ 
turing establishments, laboratories and similar industrial work. 
For commercial lighting in such places as railway stations, 
office buildings, stores, auditoriums, restaurants and school 
rooms, glass reflectors are generally employed. They are also 
commonly used in residences. Both the prismatic and the 
white-glass reflectors are used in commercial lighting. White- 
glass reflectors are made in a large variety of attractive de¬ 
signs. Prismatic reflectors are generally given a velvet finish.* 
The white-glass reflectors are superior in appearance to the 
prismatic reflectors and are therefore used, in spite of a lower 
efficienc}', where the decorative effect is important. Fig. 22 A 
shows the prismatic reflector and Fig. 22 G a white-glass re¬ 
flector of simple design. For outdoor lighting, metal reflectors 
of a weather-proof type are almost invariably used. 

89. Diffusing Globes. For direct systems diffusing globes 
may be made either of prismatic glass (Fig. 22 B), white glass 
(Fig. 22 F), or of ground glass. They are usually employed 
instead of reflectors with gas-filled tungsten lamps, because 
of the great brilliancy of the lamp. Fig. 22 ( B ) and ( F) shows 
the types used for commercial lighting in stores, offices, etc., 
and Figs. 28 and 63 the types used for outdoor service and 
industrial lighting. Since diffusing globes absorb a large amount 
of light, they are less efficient than reflectors for direct illumina¬ 
tion. Globes made of a special kind of blue glass are sometimes 
used with gas-filled tungsten lamps, to give a light very closely 
resembling daylight. This is accomplished by absorbing a 
large proportion of the light produced by the lamp. This 
results, therefore, in very low efficiency and limits the use of this 
arrangement to color-matching, illumination of store windows 
and other special purposes, as the low efficiency would prohibit 
its use for general illumination. Sometimes, instead of a spe- 

* See paragraph 83. 


TAR. 90 ] 


REFLECTORS FOR SEMI-INDIRECT SYSTEMS 61 


cial globe, a blue-glass plate is used, the lamp being mounted 
above the plate and backed by a metal reflector. (See Fig. 
22 H .) 

Accessories for Semi-indirect Lighting Systems 

90. Types of Reflectors. In the semi-indirect system, 
since the greatest proportion of the light is reflected from the 
walls and ceiling, the reflectors are always designed to entirely 
shield the lamp from view. The reflectors may take the form 
of a translucent glass bowl, a glass plate or even an ordinary 
direct type of glass reflector inverted so as to throw most of the 
light against the ceiling. The reflectors serve not only to direct 
the light against the walls, but also to diffuse that portion of 
the light which passes downward. This results in a greater 
diffusion of the light than in the direct system, with a resulting 
softening of shadows, and better uniformity of illumination 
in all parts of the room. 

91. Distribution of Light. With the usual types of semi- 
indirect reflectors, about 80 per cent of the total light is directed 
above the horizontal plane, the balance passing downward 
through the reflector to the working surface. It is apparent 
that the light which is directed above the horizontal must be 
reflected either by the ceiling or walls before it can reach the 
objects to be illuminated. Surfaces ordinarily employed for 
walls and ceilings reflect from 80 to 20 per cent of the light, 
the higher value being for white or slightly tinted surfaces, and 
the lower figure for green or brown surfaces. It can be seen 
therefore that there is a considerable loss in efficiency when 
the semi-indirect system is used. Compared with the direct 
system, about 50 to 60 per cent more energy is required for 
the semi-indirect system, where the ceiling is light in color. 

92. Translucent Bowls. The usual form of semi-indirect 
reflector consists of a translucent bowl hung directly below the 
lamp *and entirely concealing it. The bowl is made of some 
form of white glass, such as alabaster, opal, etc., which glows 
with a very pleasing, soft light when the lamp is lighted. Fig. 
22 (C) and ( E ) show this type of reflector. The bowl reflector is 
especially affected by dust, which rapidly collects on the inside 
surface, and thus reduces materially the efficiency of the unit 
and impairs its appearance, unless frequently removed. 


62 


LIGHTING ACCESSORIES 


[CHAP. 5 


93. Applications. The bowl-type of semi-indirect unit finds 
its principal application in rooms where specially artistic effects 
are required, such as restaurants, hotel lobbies, corridors, libra¬ 
ries, museums, ball rooms, churches, residences and offices. 
Gas-filled lamps are especially well adapted for use with this 
system, because of their high efficiency. The semi-indirect 
system is now used very extensively for commercial lighting 
owing to the recent improvements in the efficiency of the tung¬ 
sten lamp. For store and office lighting it is particularly 
favored, because of the better appearance of the installation. 

Accessories for Indirect-lighting Systems 

94. Types of Reflectors. With the indirect system, the lamp 
is entirely shielded from view and all the light reaches the work¬ 
ing surface by reflection from walls or ceiling. Silvered glass 
reflectors contained in bowls made of metal or plaster are 
frequently used. In some cases, the bowl is made of semi¬ 
transparent glass, which is rendered slightly luminous by means 
of small lamps, with the object of improving the appearance 
of the fixture. Fig. 22 D shows one style of indirect bowl. 
A form of indirect lighting called cove lighting has been em¬ 
ployed to some extent. With this arrangement, the lamps are 
concealed behind a cornice or cove located near the ceiling 
of the room. The efficiency of cove lighting is very low, as 
only 20 to 35 per cent of the light, produced reaches the working 
area. For The bowl type of indirect system, the useful amount 
of light is greater, being 25 to 45 per cent of the total. These 
values do not take into account the effect of dust, which rapidly 
decreases the effectiveness of the indirect system unless the 
units are frequently cleaned. 

95. Distribution of Light. The efficiency of the indirect sys¬ 
tem depends principally upon the reflecting efficiency of the 
ceiling and walls. It is apparent, from the discussion of this 
question in connection with the semi-indirect system, that the 
efficiency will be low when compared with the direct system. 
Even when using bowls with silvered reflectors, and light-colored 
ceilings and walls, the indirect system requires from 50 to 75 
per cent more power than the direct system. 

96. Applications. The indirect system is used for lighting 
drafting rooms, and occasionally for offices, stores and similar 


PAR. 97] 


ARC LAMP ACCESSORIES 


63 


places, but it is not so extensively used as the semi-indirect 
system. The objections to the indirect system are its low 
efficiency and lack of contrast in the illumination.* As a 
result, the indirect system requires a greater intensity of illu¬ 
mination than the direct system for the same kind of work. 


Arc-lamp Accessories 

97. Globes. These may be either clear glass, ground glass, 
or some form of white glass. Where the proper operation of 
the arc requires an enclosing globe, as for example, the ordinary 
enclosed carbon arc and the flame-arc, a clear inner globe is 
usually employed. An additional outer globe is used in many 
cases, either to shield the inner globe or to diffuse the light. 
Enclosed arc lamps for outdoor service usually employ clear 
outer globes, while for interior lighting a diffusing globe is gen¬ 
erally employed to reduce the glare. Flame-arc lamps, in 
general, require diffusing outer globes of white glass, because of 
the intense brilliancy of the arc. 

98. Reflectors. Direct-current enclosed arc lamps have a 
natural downward distribution of light (see Fig. 5A ) and there¬ 
fore reflectors are not usually required. A.c. arc lamps, since 
their distribution is wider (Fig. 5B) usually require reflectors 
for efficient use of the light. The flame-arc lamps have a wide 
distribution (Figs. 7 A and B), but since they are high candle- 
power units which must light a large area the natural distribu¬ 
tion is usually satisfactory. Reflectors are sometimes provided 
for industrial lighting so as to concentrate the light inside 
the 60° zone. (See Fig. 8.) Metallic-electrode arcs are not 
usually provided with reflectors, as the natural distribution 
(Fig. 10) is well adapted to the lighting requirements. A refrac¬ 
tor, which serves the same purpose as a reflector, is, however, 
used with one type of metallic-electrode arc lamp (see Fig. 
9a). The reflectors for enclosed carbon arc lamps may be 
either porcelain, enameled-steel, or white glass. The steel 
reflectors will withstand rough usage and are preferable, except 
where it is desired to illuminate the ceiling of a room. 


* See paragraph 72, 


CHAPTER 6 


LIGHTING FIXTURES 

99. Types of Fixtures. The term “ lighting fixture ” is used 
to designate the necessary supporting device which is required 
for properly mounting an incandescent lamp and its reflector. 
The fixture therefore includes the lamp-socket or receptacle,* 
suitable supports for this socket and the reflector. The entire 
combination of lamp, reflector, socket and supporting devices 
is called a light-unit. In some cases, supporting fixtures are 
employed with arc lamps and mercury-vapor lamps, but the 
following discussion applies only to fixtures used with incan¬ 
descent lamps. Fixtures are of two general classes—ceiling 
and wall fixtures or brackets. Each class of fixture may be 
of several types; direct, indirect, or semi-indirect, depending 
upon the system of illumination for which it is designed. The 
fixture is subject to wide variations in design, since under this 
name is included all types, from the single-lamp pendant 
fixture or “ drop,” costing possibly a dollar, to the highly deco¬ 
rative fixtures containing many lamps and costing thousands 
of dollars. It is apparent that the fixture should support the 
lamp and reflector in such a position as to distribute the light 
in the proper direction. Modern commercial fixtures are care¬ 
fully designed with this object in view. In the more artistic 
fixtures, however, efficiency is often sacrificed for the sake of a 
more decorative effect. 

Fixtures for Direct Lighting 

100. Single-lamp units. The simplest type of unit is the 
ordinary pendant or “ drop,” consisting of a socket with suit¬ 
able reflector, a length of flexible twin conductor and a “ rosette ” 
adapted for attaching to the ceiling. Such a unit is shown in 
Fig. 24. For this fixture a socket tapped for f-in. pipe should 

* See Chapter 1G. 

64 


PAR. 100] 


TYPES OF FIXTURES 


65 


be used, and the conductor should be 11 reinforced cord ” * 
in stores, etc., where not subject to rough usage. For factories, 
packing-house cord ” should be used. For lamps of 150 
watts or less, No. 18 cord is satisfactory. For larger lamps, 
No. 16 or 14 cord should be used. The style of rosette used 
depends upon the system of wiring. A plain type of unit 
for use with conduit systems and suitable for industrial light¬ 
ing is shown in Fig. 25. This consists of a one-piece steel 



Fig. 24.—Cord 
Pendant. 



Fig. 25.—Rigid Fixture, Industrial 
Type. 


reflector and a length cf J-in. iron conduit. Connection is 
made with the conduit system by means of a T-connection. 
This is used for exposed conduit work. For concealed work, a 
steel or cast-iron outlet boxf would be used. If glass reflectors 
are used with this type of unit, a keyless socket and suitable 
reflector holder are provided in place of the combined reflector 
and socket shown in the illustration. These fixtures are usually 
wired with regular No. 14 wire carried directly to the socket. 
A rigid fixture of a more ornamental type is shown in Fig. 26a. 
This is adapted for concealed wiring systems and employs 

* See paragraph 260. . t See paragraph 227. 






















66 


LIGHTING FIXTURES 


[chap. 6 


polished brass tubing instead of the iron conduit. A “ canopy ” 
(1) is used to cover the connections to the outlet box. In some 
cases these fixtures are made flexible by adding a link near 
the canopy (Fig. 22/). Fixtures of these kinds are usually 
wired with fixture wire or flexible cord. Another form of single- 
light unit is shown in Fig. 266, where an ornamental chain is 

used instead of the tubing. 
This fixture is wired by means 
of flexible cord, which is colored 
to match the finish of the fix¬ 
ture, and is carried down the 
outside of the chain. Units of 
the type shown in Fig. 24 can 
be used where the lamp is 
located within reach and is to 
be controlled by means of a 
key socket. The same type 
of unit is also used with key¬ 
less sockets (as shown in the 
figure), where the lamps are 
mounted out of reach, but 
where an inexpensive installa¬ 
tion is desired. The rigid type 
of fixture (Figs. 25 and 26a) 
is adapted for use only where 
there is no danger of the unit 
being struck. These fixtures 
should never be mounted so 
low that they are within reach 
from the floor. The more orna¬ 
mental types (Figs. 26a and 6) 
are used for commercial light¬ 
ing in offices, stores, resi¬ 
dences, restaurants, etc. For 
low ceilings, the unit may consist of a suitable receptacle 
with reflector (Fig. 27a). The dome type (Fig. 276) is used 
for corridors and similar places. Shock absorbers were used 
extensively at one time on fixtures containing tungsten lamps, 
to reduce the filament breakage. The present form of wire¬ 
drawn lamp is, however, so rugged that shock absorbers 
are no longer necessary even where rigid fixtures are used. For 



Fig. 26. —Single-lamp Units. 
Commercial Types. 

1. Canopy. 







































PAR. 101 ] MULTIPLE-LAMP UNITS 67 

\ 

installations where excessive vibration occurs, a flexible unit 
as shown in Fig. 24 is all that is necessary. 

101. High Candlepower Units. The extensive use of large 
gas-filled tungsten lamps has called for specially designed light 
units. Reflectors are not used for these units unless they can 



a . Reflector. b . Dome. 

Fig. 27.—Units for Low Ceilings. 


be mounted high enough to be entirely out of the range of vision. 
It is more common to use diffusing globes* in order to shield 
the eye from the bright filament. For commercial lighting, 
the unit may be of the type shown in Fig. 22 B and F. For 
industrial lighting, a combination of 
enclosing globe and steel reflector is 
often used.. Such units are shown 
in Figs. 28 and 63. The unit shown 
in Fig. 63 is of a particularly sub¬ 
stantial type and is well adapted for 
outdoor service. All of these units 
are thoroughly ventilated. A semi- 
indirect unit which can be used 
where the ceiling is dark, or where 
there are skylights, is shown in 
Fig. 29. This provides a suitable 
reflecting surface above the enclos¬ 
ing bowl, so as to properly direct the light down. 

102. Multiple-lamp Units. The multiple type of unit was 
more commonly used when the 16-cp. carbon lamp was the 
standard unit for incandescent lighting. At present, when 

* See paragraph 89. 



Tungsten Lamp. 























































68 


LIGHTING FIXTURES 


[CHAP. 6 


tungsten lamps may be obtained in sizes ranging from 8 to 
1700 cp., the tendency is to use a single lamp for a fixture. 
From an efficiency standpoint, it is better to use single units hav¬ 
ing one large lamp rather than multiple units with several small 
lamps. Thus, if the proper illumination of a room 'required 
the use of about 100 watts for each fixture, it would be better 
in general to use a 100-watt lamp, with the proper reflector, 
than to use four 25-watt lamps with separate reflectors. Mul¬ 
tiple fixtures are made 
in a great variety of 
attractive designs and 
are chiefly used for 
installations where 
artistic effects are im¬ 
portant and efficiency 
is secondary. There 
is a tendency, however, 
to use multiple fixtures 
in many cases, where a 
suitable single lamp 
unit would be more 
satisfactory both from 
the standpoint of 
efficiency and of dec¬ 
orative effect. Mul¬ 
tiple fixtures may take 
many forms, from the plain vertical stem with two or more 
branching arms at the bottom, to the enormous and costly 
fixtures or “ chandeliers ” used in lighting auditoriums, 
railroad waiting-rooms, etc. The modern multiple fixtures 
of plain types are designed with considerable attention to 
efficiency, with the lamps pointing vertically downward, 
and each provided with a high-efficiency reflector. The old 
design having the lamps projecting at an angle should not be 
used. Fig. 30a shows a modern type of fixture of good design. 
For fixtures of this type, keyless brass sockets for medium-base 
lamps would be used. Another type of small multiple fixture 
is the shower type shown in Fig. 306. These fixtures usually 
have the lamps closer to the ceiling than the branch type (Fig. 
30a). It is not usual to employ lamps larger than 50 watts in 
either of these types of fixtures. Glass reflectors or decorative 



Fic,. 29.—Semi-indirect Unit. 

Used where the ceiling is dark. 



















PAR. 103] 


MULTIPLE-LAMP UNITS 


69 


shades are used with the shower type of fixture. Large multiple 
fixtures take so many forms that it is impossible to describe 
them in detail. The lamps are usually of small size, 25 or 50 
watts, and no reflectors are employed. Generally the lamps 
are frosted to reduce their brilliancy. In some large fixtures, 
ground glass or opal balls are used, each enclosing a higli- 



Fig. 30.—Multiple-lamp Units. 


candlepower tungsten lamp. The wiring for multiple fixtures 
is generally concealed between the central supporting pipe and 
the ornamental enclosing shell, which is usually metal tubing. 

103. Applications. Plain fixtures, such as are shown in Fig. 
30, are best adapted for residences, offices, stores, small rooms 
in railway stations and similar places, although it should be 























































































70 


LIGHTING FIXTURES 


[CHAP. 6 


remembered that single-unit fixtures might be used for many 
of these places with as good or better results. Large fixtures 
are usually specially designed for a particular installation, and 
find their chief application in the illumination of auditoriums, 
public rooms in railway stations, libraries, museums, and large 
private residences. 

104. Wall Brackets. Wall brackets are used to supplement 
the general illumination provided by the ceiling fixtures and to 
give a local illumination for special purposes. They also serve 
in some cases to improve the decorative appearance of an instal¬ 
lation. Fig. 31 shows two types of wall brackets. Usually 
these brackets are equipped with ornamental shades or else 




Fig. 31.—Wall Brackets. 

the lamps are frosted and used without shades or reflectors. 
Wall brackets are chiefly used in residences, offices, restuarants, 
etc. With modern systems of lighting there is very little 
occasion for their use. 

Fixtures for Semi-indirect Lighting 

105. The usual semi-indirect fixture is the bowl type, examples 
of which are shown in Figs. 22(7 and E. This type of unit con¬ 
sists of a white-glass bowl of high reflecting power supported 
directly below one or more tungsten lamps and entirely con¬ 
cealing them. The glass of which the bowl is made is semi¬ 
transparent, so that only a small amount of the light (about 
15 or 20 per cent) passes through it, the greater proportion 
being reflected upon the ceiling. The units shown in the illus¬ 
tration employ a single lamp, but in some cases several lamps 



















PAR. 106 ] FIXTURES FOR SEMI-INDIRECT SYSTEMS 


71 


are used. The choice between one or several lamps depends 
upon the design of the bowl. Another design is shown in 
Fig. 32. This employs a wide metal band which is backed 
by a mirror reflector. The central bowl is of white glass, which 
transmits a portion of the light and reflects the rest upon the 
ceiling. An inexpensive semi-indirect unit is obtained by using 
fixtures similar to Fig. 30a with the 
sockets and reflectors turned towards 
the ceiling. By this means, the greater 
part of the light is thrown upward in¬ 
stead of downward as would be the case 
with the reflectors in the normal posi¬ 
tion. This type of fixture has been 
successfully used in drafting rooms 
and offices. 

Fixtures for Indirect Lighting 

106. The usual type of indirect unit 

is similar to the bowl type, semi-indi¬ 
rect fixture, the chief difference being 
that the bowl is opaque. As was ex¬ 
plained in paragraph 94, cove lighting 
was at one time used to some extent, 
but it is so inefficient that, at present, ^ IG - 32. Semi-indirect 
the bowl-type fixture has taken its Unit. 

place. The bowl is made of metal or 

plaster, and contains a suitable reflector, which, in the best 
types, is silvered glass. Either one large lamp or several 
small lamps may be used, depending upon the design of the 
bowl. Fig. 22 D shows an efficient type of indirect unit. 

107. Insulating Joints. Usually the insulation of the fixture 
wiring is considerably weaker than that of the remainder of the 
system. This is due to the design of the sockets and the 
weakness of the fixture wire used for connecting them to the 
circuit. This wire is not only smaller, but has a thinner insula¬ 
tion than that used on the rest of the circuit. Fixture wire 
must be used in most units, because of the small space avail¬ 
able for running the wire. Short-circuits and grounds are 
therefore most likely to occur in the fixture wiring or in the 
socket. It is therefore desirable to localize trouble due to 
these defects by insulating the fixtures from the grounded 


























72 


LIGHTING FIXTURES 


[CHAP. 6 


conduit system, gas pipes or other grounded metal objects. 
The insulating joints used for this purpose are couplings hav¬ 
ing threaded ends separated by mica or moulded insulation. 
Fig. 33 shows the construction of such a joint. The joint 
screws on to the end of the fixture and attaches to a threaded 
stud which is secured to the ceiling or wall. Where gas and 
electric fittings are combined in the same fixture, the insulating 
joint has an opening through the centre (Fig. 336) for the 
passage of the gas. If the fixture is used simply for electric 
lamps, a solid joint of similar design is used (Fig. 33c). Red 


Insulation 



s 

Fig. 33.—Insulating Joints for Fixtures. 

b. For combination fixtures, c. For straight electric fixtures. 

lead or graphite should never be used when connecting an 
insulating joint, since these substances are conductors of elec¬ 
tricity and are likely to spread over the insulating surfaces 
and ground the fixture. Asphaltum paint is the most satis¬ 
factory material to use for this purpose. Insulating joints 
must be used * where fixtures are supported from outlets of 
systems using metal conduit, armored cable, or metal moulding, 
or when supported from gas piping or metal work. They are 
also required when fixtures are installed on metal walls or 
ceilings, or on plastered walls containing metal lath or on walls 
or ceilings of fireproof buildings. If suitable keyless sockets 
are used and No. 14 rubber-insulated wire is run directly to the 
sockets, the insulating joint may be omitted. No insulating 
joint is required, for example, when the fixture shown in Fig. 
25 is used. Canopies (Fig. 26) are provided to cover the con¬ 
nections to the fixture and the insulating joint, if one is used. 
Canopies for fixtures having insulating joints must be provided 
with an insulating ring at the top, to keep the metal out of 
contact with the ceiling or wall. (Fig. 35.) 

* Rules of the “National Electrical Code.” 




























































PAR. 108] 


SUPPORTS FOR FIXTURES 


73 


108. Methods of Supporting Fixtures. A substantial sup¬ 
port for a fixture is important, particularly for large fixtures 
which may weigh several hundred pounds. All single-lamp 



Joists 

•Split Knobs. 


Flexible Tubing 


Crowfoot 


Hickey- 


Plaster 


Canopy (Cut away to 

show connections) 

-'Brass Tubing 


Cleat Nailed 
to Joists 


Fixture Wire 


Fig. 34.— Support for Fixture. 

Knob and tube wiring in frame house. 



Fig. 35.—Support for Fixture using Outlet Box. 

Arrangement shown is for armored cable. A similar method is used for rigid 
or flexible conduit. 

fixtures and the ordinary sizes of multiple fixtures can be sup¬ 
ported from |- or |-in. pipe fittings. Fig. 34 illustrates a satis¬ 
factory method of support for concealed “ knob and tube ” 







































































74 


LIGHTING FIXTURES 


[CHAP. 6 


wiring. A f-in. strip nailed to the floor timbers holds the 
flexible tubing covering the wires and supports the fixture by 
means of a “ crow-foot.” Connections are made to the fix- 




Fig. 36.—Support for Combina- Fig. 37.—Support for Combina¬ 
tion Fixture. tion Fixture. 

Knob and tube wiring. For conduit systems. Wires not shown. 


ture wires at a point just below the ceiling line and the whole 
covered by the canopy. Where it is not possible to install a 
strip as shown (for example in the wiring of an old building) 


Yi Conduit about 12 long 



Fig. 38. —Fixture Support for Concrete Ceiling. 

Wires not shown. 


a wooden base block not less than f in. thick may be fastened 
to the surface of the plaster. This arrangement could also 
be used for moulding work. Systems using rigid, or flexible 




























































PAR. 108] 


SUPPORTS FOR FIXTURES 


75 


conduit, or armored wire are always provided with steel or 
cast iron outlet boxes * for each fixture. These boxes serve 
as a termination of the conduit, allow space for splicing the 
wires to the fixture circuits, and also in some cases provide 
a support for the fixture. In non-fireproof buildings, the boxes 



Fig. 39. —Fixture Support for Concrete Ceiling. 

Conduit cast in. 


may be screwed to a block nailed between the floor timbers in 
order to put the lower edge of the box flush with the finished 
plaster line. (Fig. 35.) The outlet box contains in the centre 
a threaded stud (Fig. 42) to which the fixture is attached. 

When a combination gas and electric fixture is installed, 


Finished Floor 


Box Hanger 


Cinder Fill 



Plaster'^ 


Shallow 
Outlet Box 


Terra-cotta 
Eire Proofing 


Fig. 40.—Fixture Support for Terra-cotta Ceiling. 


arrangements like Figs. 36 or 37 are used. In Figs. 38 and 39 
are shown suitable supports for outlets in fireproof buildings, 
where the plaster is placed directly on the bottom surface of 
the floor slab. The weight of the fixture is not carried by the 
box, but by a length of conduit which passes through the slab 
and terminates in an eye and a short length of conduit forming 

* See paragraph 227. 




































































76 


LIGHTING FIXTURES 


[chap. 6 


a cross-bar. This is concealed under the finished floor. The 
outlet box is supported from the conduit holding the fixture 
by means of the lock units. The arrangement in Fig. 38 uses 
a shallow box which is placed directly against the floor slab. 



The other construction (Fig. 39) employs a deep box, which is 
set in the concrete, the latter method is more commonly 
used where the box and conduit are installed on the forms and 

the concrete cast around them. When 
a shallow box (Fig. 38) is used a wooden 
plug can be placed in the form to pro¬ 
vide an opening for the installation 
of the conduit after the concrete is in 
place. If desired, however, the conduit 
can be installed before the concrete 
is placed, as in the other construc- 
Fig. 42. 4ixture Stud. tion. Fig. 40 shows an arrangement 

for floors built of hollow tile. Where 
a “ hung ceiling ” is used, an arrangement similar to Fig. 
41 may be used. A deep box is employed and the conduit 
enters on the side instead of the top. The supporting pipe 
may be \ in. or 4 in. for large fixtures. The types of supports 










































PAR. 108] 


SUPPORTS FOR FIXTURES 


77 


shown in Figs. 38 to 41 are very satisfactory for fixtures of 
the usual sizes. For very small fixtures, a threaded stud 
(Fig. 42) may be used. This is attached directly to the outlet 
box, which must be securely fastened to the ceiling. Very 
large fixtures are supported from a chain which is attached to 
a suitable beam framed into the steel work of the building. 
These fixtures are usually located so high that they cannot be 
reached by ladders from the floor. Arrangement must there¬ 
fore be made to lower the fixture for cleaning or renewal of 
lamps. Wall fixtures or brackets can be supported from studs 
(Fig. 42) in the outlet box unless they are very heavy, when they 
must be bolted to the wall. 


CHAPTER 7 


PRACTICAL METHODS OF CALCULATING INTERIOR 

ILLUMINATION 

109. The Problem of Interior Illumination. At the present 

time the advantages of a satisfactory lighting system are very 
generally appreciated. Since, however, the wiring is usually 
installed in iron conduit, and is frequently concealed in the walls 
and ceilings, proper attention should be given at the start to 
the design of the lighting system to avoid expensive changes 
after the system has been put in operation. For some places, 
such as libraries, museums, theatres, large residences and 
high-class stores, the lighting system should harmonize with the 
furnishings and furniture and the illumination should assist 
in producing the desired decorative effect. Installations of 
this kind require individual treatment by lighting experts and 
will not be considered here. For industrial lighting and many 
cases of commercial lighting, efficiency and cost of the installa¬ 
tion are of first importance, and decorative effects, while they 
should not be entirely neglected, are of secondary interest. 
Standard designs of reflectors and fixtures can be used in such 
cases, and by using data from similar installations it is com¬ 
paratively easy to secure a satisfactory lighting installation. 
Only this kind of lighting problem will be considered here. 
The factors which are involved in planning a lighting system 
are: 

1. Class of lighting , whether industrial or commercial. This 
obviously depends upon the use which is made of the illumi- 

m i 

nation. 

2. Method of illumination , whether localized, general or a 
combination of the two. 

3. System of illumination, whether direct, indirect or semi- 
indirect. 

4. Type of lamp to be used, whether arc, incandescent or 
vapor tube. 


78 


PAR. 110] 


METHOD OF ILLUMINATION 


79 


5. Intensity of illumination , which depends upon item (1). 

6. Power Required. This is affected by all of the foregoing 
items. 

7. Size of Light Unit. This depends upon the size of the 
room. 

8. Location of light uniis, which requires consideration of 
the proper spacing and height of mounting. 

9. Reflectors or Globes. 

(1) Class of Lighting 

110. Before a lighting system can be intelligently designed, 
information regarding the kind of work carried on must be 
secured. The location of machines or furniture should be deter¬ 
mined as far as possible, and the height of ceiling and size of room 
must be known. The color of walls and ceiling should also 
be considered. Based on the character of the work, the light¬ 
ing may be classified as either industrial or commercial. Indus¬ 
trial lighting includes all applications where efficiency of illu¬ 
mination is of the greatest importance, and where decorative 
effect and appearance of the lighting units is relatively 
unimportant. In this class would be included factory and 
warehouse lighting. Commercial lighting includes appli¬ 
cations where more attention must be paid to securing an 
installation which will harmonize with the decorations and 
furnishings of the rooms. In such places, efficiency, by which 
is meant the relative power consumption, may be, to a certain 
extent, sacrificed for the sake of improved appearance. This 
is secured usually by careful attention to the proportion of 
the fixtures, and to the effect of the illumination upon the 
decorations of the room. To this class belongs the lighting 
of office buildings, stores, restaurants, hotels, libraries, muse¬ 
ums, railway stations, residences, etc. 

(2) Method of Illumination 

111. Localized lighting was at one time very commonly 
employed and is in fact the simplest way of utilizing the light. 
With this method an incandescent lamp of small candle- 
power is provided for each worker or machine, and no attempt 
is made to provide general illumination of the room. An 
arrangement of this kind is necessary in some cases, for example 


80 CALCULATING INTERIOR ILLUMINATION [CHAP. 7 

when machining the interiors of castings or boilers, or in other 
places where a general illumination could not penetrate. It 
can also be used to light small areas which are widely sepa¬ 
rated. An installation of local lighting is likely to be expensive 
to install owing to the large number of units employed, and 
expensive to maintain because of the chance for damage to the 
units by careless handling. Furthermore a change in the 
location of machines would usually require expensive changes 
in the lighting system. The use of this method should there- 



Fig. 43. —Example of Localized Lighting. 

(National Lamp Works of G. E. Co.) 

fore be confined to places where general illumination cannot 
be used, or where it can be combined with a moderate general 
illumination to produce a high light intensity for special pur¬ 
poses. The second of these two applications is more commonly 
found at present (see Fig. 43). 

112. General Lighting. In this method practically uniform 
light intensity is produced over the entire room. The light units 
are equipped with efficient reflectors to properly control the 
light and are evenly spaced, without regard to the location of 
machinery or furniture (Fig. 44). Before the introduction of 
tungsten lamps only arc lamps were used for general illumina- 




PAR. 113] 


METHOD OF ILLUMINATION 


81 


tion because of the large amount of power required when carbon 
lamps were used. General lighting is best adapted to large 
rooms free from obstructions, where the workers are located 
close together, so that the same illumination intensity is 
required over practically the entire room. It is also well 
adapted for stores, offices and similar places, where there are 
possibilities of frequent rearrangement of the furniture, etc. 



Fig. 44.—Example of General Lighting. 

Installation of 250-watt lamps with 16-foot mounting height. (General 
Electric Co.) 

High-candlepower tungsten lamps, arc lamps, and mercury- 
vapor lamps may all be used for general lighting. 

113. Localized general lighting or group lighting is a modi¬ 
fied form of general lighting. High-efficiency light units are 
used, but they are not uniformly spaced and no attempt is made 
to secure uniform illumination. Instead, the units are spaced 
with particular reference to the machines, thus providing the 
proper intensity at the working points and a lower intensity 
at other places. This method is particularly adapted for 
rooms where there are a number of similar machines, arranged 












82 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 



in rows, as in machine shops, weave rooms, etc. (Fig. 45). 
The same kind of units may be used for this method as for gen¬ 
eral illumination. Group lighting is more economical than 
general lighting where a high intensity is required only over 
the machines. There is of course no sharp dividing line 
between the group system and the general lighting method, 
since a particular installation may tend towards one or the 
other, depending upon the arrangement of the units. In some 


Fig. 45.— Example of Localized General Lighting. 

Society )^ reflection from the goods to the ceiling. (Illuminating Engineering 

cases a combination of general and localized lighting is desir¬ 
able. For this method sufficient general lighting is provided 
to illuminate distinctly the various objects in the room while 
lamps of small size are provided for each worker. Such an 
arrangement is desirable where a very strong light is required 
upon the work and where it would be difficult to secure this 
by the general lighting without producing troublesome shadows. 
I his method is particularly useful in lighting sewing machines 
where each worker is provided with a small tungsten lamp in 
a metal reflector located on a flexible arm which can be adjusted 








PAR. 114] 


SYSTEMS OF ILLUMINATION 


83 


to throw a strong light directly upon the work. This arrange¬ 
ment can also be used to advantage for fine machine work, 
inspecting, assembling, office work, etc. The particular 
method to use depends somewhat upon the requirements of the 
installation. Tables 8 and 9 indicate the usual practice. 

(3) Systems of Illumination * 

114. The direct system is usually employed for industrial 

lighting, since it requires the least power to produce a given 
amount of illumination. Occasionally, however, an indirect 
system is used for work rooms with low ceilings, or where shadows 
must be eliminated, as in drafting rooms. The direct system 
is also used in many commercial installations where efficiency 
is of great importance. 

115. The semi-indirect system is at present used very 
extensively for commercial installations where a reasonably 
high efficiency is desired, but where it is also necessary that the 
light units shall harmonize with the furnishings of the room. 
The numerous designs of semi-indirect bowls and supporting 
fixtures now on the market allow the use of units which are not 
only efficient but are also effective in improving the appearance 
of the installation. The principal applications of this system 
are in the better classes of stores, restaurants, residences, hotels, 
etc. 

116. The indirect system finds only a limited use, for draft¬ 
ing rooms, hotel bed rooms, etc. Where a direct system is 
not used, the semi-indirect system is in general preferable to 
the indirect system, both because of the slightly better efficiency 
of the latter and because of the better appearance of the instal¬ 
lation. Neither of these systems can be effective unless the 
ceiling is light in color. The particular applications of these 
systems are indicated in Tables 8 and 9. 

I 

(4) Type of Lamp f 

The size of room, height of ceiling and character of work 
performed all have a bearing upon the selection of the type of 
lamp. Tungsten lamps are now made in sizes to suit all kinds 
of commercial and industrial lighting requirements and are used 

* See paragraph 68. 

t See Chapters 2 ancf 3 for descriptions of lamps. 


84 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


much more extensively than any other kind of lamp. They 
are practically the only type now used for commercial lighting. 
Flame-arc lamps are especially well suited for use in smoky 
places or dusty locations because of the penetrating quality 
of the light, and hence they have been used extensively in steel 
mills, foundries, etc. The gas-filled lamps are, however, now 
being used in these places and there is a tendency at present to 
use them in place of flame-arcs. Mercury-vapor lamps are well 
adapted for some kinds of industrial lighting because of the 
color of the light. In machine shops and where varnishing 
and finishing work is done, they seem to be especially desirable. 

(5) Intensity of Illumination 

117. The importance of a suitable intensity has been dis¬ 
cussed in paragraph 70. There is a decided tendency at present 
to use rather high intensities. In industrial establishments 
there are good arguments for such a practice because it increases 
the amount of finished product, decreases the quantity of 
spoiled work and reduces the number of accidents. Due to 
the importance of this subject a “ Code of Lighting ” has 
recently been issued * to serve as a guide for factory lighting. 


Approximate Illumination Intensities for Industrial Lighting. 


Class of Work.' 

Foot-candles. 



1 . . . 

Minimum. 

Desirable. 

Storage, passageways, stairways and the like 

0.25 

0.25- 0.5 

Rough manufacturing and other operations. . 

1.25 

1.25- 2.5 

Fine manufacturing and other operations.. . . 

3.50 

3.50- 6.0 

Special cases of fine work. 

.... 

10.00-15.0 


These values are the average intensities on a horizontal plane 
on a level with the work. The intensities given are only gen¬ 
eral values and for a particular problem more definite informa¬ 
tion is required. Tables 8 and 9 give data of this kind for 
lighting with tungsten lamps. The operating cost of a suitable 
lighting system is only about § of 1 per cent of the total oper¬ 
ating costs for industrial establishments and only about 1 per 
cent of the total sales in the case of stores, and hence is a very 
* Illumination Engineering Society, 1915. 













PAR. 118] 


INTENSITY OF ILLUMINATION 


85 


small item in the total cost Of doing business. For stores and 
office buildings the lighting system serves as one form of adver¬ 
tising. There are therefore very good arguments for using a 
satisfactory lighting system. 

118. Choosing a Suitable Intensity. It will be noted that 
in both Tables 8 and 9 a range of intensities is indicated. 
Where first cost of installation is important or where the 
character of the work is such that the highest intensity is not 
required, the lower value should be used. The intensity used 
may be influenced considerably by certain peculiar conditions 
of the installation, and the tables should not be used blindly 
without taking these into account. Thus it is found that an 
intensity which is entirely sufficient for proper vision, if used 
alone, may be insufficient when supplemented by daylight. 
If therefore the artificial lighting system is to be used when 
daylight is also employed (for example late in the afternoon) 
the intensity must be increased somewhat. It is also apparent 
that the illumination required for work on dark material is 
greater than is required for light material. The intensity 
necessary for local lighting is usually greater than that where 
general lighting is used. This is due to the strong contrasts 
in intensity which are likely to exist when local lighting is used. 
It is always well to be somewhat liberal in choosing the illumi¬ 
nation intensity, since the tables are based on clean lamps and 
reflectors. An allowance should therefore be made for depre¬ 
ciation due to dust. The amount of this depreciation is indi¬ 
cated in paragraph 87. Where conditions are such that the 
reflectors may be regularly cleaned, an allowance of at least 
10 per cent excess illumination should be made, while for instal¬ 
lations which will probably not be cleaned regularly, as much 
as 25 per cent excess should be allowed. In Tables 8 and 9, 
where general illumination ( G ) is indicated, the specified inten¬ 
sity is the average over the entire room on a horizontal surface 
at the usual height of the work. This surface is called the 
working plane. Usually this is taken as 30 in. above the floor. 
This illumination is secured by light-units uniformly spaced 
over the entire room. Where local illumination (L) is specified,' 
the intensity given is that at the point on the machine to which 
the light is directed. Where combined local and general illu¬ 
mination (G and L) is specified, the value given is for the gen¬ 
eral illumination only. The amount of local illumination is 


86 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


indicated by specifying a definite size of lamp. Where group 
lighting (L-G) is used the values are those on the work. This 
is produced by lamps located with particular reference to the 
machine, and recommendations for the size of these lamps are 
given in the tables. If uniform illumination is desired instead 
of group lighting, the values of foot-candles in column 3 of 
Table 9 can be used as a guide. 

119. Method of Securing Uniform Illumination. If general 
lighting (giving uniform illumination) is to be provided by means 



^ Working Plane 

Floor Line 



Fig. 46.—Illumination Produced by a Single Tungsten Lamp. 
(100-watt Mazda B lamp with intensive, prismatic reflector.) 


of tungsten lamps, there is a definite relation between the spac¬ 
ing and height of units and the style of reflector used. Consider 
first a single lamp equipped with a reflector for direct lighting. 
Fig. 46 shows the intensity on a horizontal surface 8 feet below 
the unit, neglecting the effect of walls and ceiling. Directly 
beneath the lamp, the intensity is 2.45 foot-candles, while 5 

































































































































































PAR. 119 ] 


UNIFORM ILLUMINATION 


87 


ft. away from this point it is 1.69 foot-candles, and at 10 ft. it 
has dropped to 0.38 foot-candle. It is apparent that in a room 
lighted by a single lamp, a wide variation of illumination inten- 


Spacing 




Fig. 47.—Illumination Produced by Two Tungsten Lamps. 

(100-watt Mazda B lamps with intensive prismatic reflectors.) Curve A. 
Illumination produced by lamp A. Curve B. illumination produced by lamp B. 
Curve C. Combined illumination. 


sity is likely to exist. If a second lamp of the same size is located 
10 ft. away (Fig. 47) the light from the two lamps overlaps 
and there is less variation in the illumination. There would 



















































































































































































































88 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


now be 2.83 foot-candles directly below either lamp, while 2 
ft. from the lamp the intensity is 3.2 foot-candles and at 5 ft. 
or midway between the lamps it is 3.3 foot-candles. By 



Fig. 48. —Effect of Height upon 
Uniformity of Illumination. 

Lamps arranged with proper height 
and spacing. Intensive style of re¬ 
flector. 


face by Tungsten Lamps. 

This shows a plan view of the lamps 
in Fig. 48. Most of the light from A 
falls on square abed, and from B falls 
on square befc. The light overlaps 
somewhat, as shown in Fig. 47. 


using a number of these units, spaced 10 ft. apart and mounted 
8 ft. above the working plane, practically uniform illumina¬ 
tion could be secured. The light which is reflected from walls 

and ceiling helps to make the 
illumination more uniform and 
increases the intensity con¬ 
siderably. A change of height 
affects the distribution. When 
the lamps are properly located 
to give uniform illumination, as 
shown in Fig. 47, most of the 
light from a lamp is directed by 
means of the reflector to the 
portion of the working surface 
beneath it. We can consider 
that each lamp serves princi- 
Fig. 50.—Effect of Height upon pally to illuminate a square 
Uniformity of Illumination. directly below the lamp (Figs. 48 

Lamps arranged with proper spa- and 49), the length of the sides 
cing but mounted too high. Intensive n 41 _ 1 • i , 

style of reflector. ^ the square being equal to 

the distance between lamps (in 

this case 10 ft.). There is, of course, a certain amount of light 

coming from lamp B for example which falls upon the square 

under lamp A. This, however, is only sufficient to increase the 



Floor Line-^ i 








































PAR. 119 ] 


UNIFORM ILLUMINATION 


89 




Fig. 51. —Illumination Produced by Lamps Shown in Fig. 50. 

Height too Great for Spacing. 

(100-watt Mazda B lamps with intensive prismatic reflector.) 
















































































































90 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 




Fig. 52. — Effect of Changing Reflectors. 

Spacing and mounting height the same as in Fig. 51, but a more concentrating 
reflector is used. (100-watt Mazda B lamps with focussing prismatic reflector.) 


















































































































PAR. 119] 


UNIFORM ILLUMINATION 


91 


illumination near the edges of the square under A so as to give 
a uniform illumination. If we use the same lamps and reflec¬ 
tors with the same distance between lamps but increase the 
height of the unit, we have the condition shown in Fig. 50. 
Here the light spreads too much and a considerable portion 
strikes the walls, where much of it i$ absorbed and lost, or is 



Fig. 53. —Method of Obtaining Uniform Illumination for Different 
. Mounting Heights. 

Style of reflector is changed as mounting height is changed. 

(Holophane Works of G. E. Co.) 

reflected away from the working surface so that only a relatively 
small part is useful. The intensity produced under these con¬ 
ditions is shown in Fig. 51. By using a reflector which con¬ 
centrates the light more, better results can be obtained. A 
focussing type reflector using the same size lamp gives the inten¬ 
sities shown in Fig. 52. It will be seen that these are nearly 
equal to the intensities produced when the lamps were 



























92 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


equipped with intensive reflectors and were mounted 8 feet 
high. The reflection from the walls would tend to equalize 
this difference and therefore the height of mounting does not 
greatly affect the illumination intensity, provided the proper style 
of reflector is used. The relation between mounting height, 
spacing and style of reflector is shown in Fig. 53 for the reflec¬ 
tors made by one manufacturer. Since there usually cannot be 
very much variation in mounting height in a particular case, 
different spacings as a rule require different styles of reflectors. 
The method of selecting a particular reflector for a certain spac¬ 
ing and mounting height is explained in paragraph 128. With 
indirect and semi-indirect systems there is also a definite rela¬ 
tion between the spacing of the units, the height above the floor 
and the distance below the ceiling, and this must be followed 
if uniform illumination is to be secured. With arc lamps 
and mercury-vapor lamps, no attempt is made to change the 
reflectors to suit different mounting heights. As used, however, 
a fairly uniform illumination is secured. 

* 

(6) Power Required—Uniform Illumination 

120. Tungsten Lamps. The area of the room, multiplied 
by the required intensity in foot-candles, will give the total 
lumens which must strike the working plane.* The lamps 
must furnish all of this light and in addition must supply the 
light which is absorbed by the reflector, walls and ceiling. 
The amount of light thus absorbed depends upon the kind of 
reflector used and the color of the walls and ceiling. Even 
under favorable conditions only 40 to 60 per cent of the total 
light produced by the lamp finally reaches the working surface. 
The percentage of the total light which is useful is called the 
utilization efficiency of the lamp. This varies with the kind 
of reflector used and the color of the walls and ceiling. Values 
for tungsten lamps are given in Table 10. Dividing the lumens 
required for the working surface by the utilization efficiency 
gives the total lumens which the lamps must produce. Dividing 
the total lumens produced by the lamps by the lumens per wattf 
for the particular size of lamp used gives the total watts neces- 

. i 

* Since 1 lumen per square foot will produce an illumination of 1 foot-candle. 
See Chapter 4. 

| Given in tables of lamp ratings. 


PAR. 120 ] 


POWER REQUIRED 


93 


sary to produce the required intensity. This can be ex¬ 
pressed by formulas as follows: 

Let A =area of room in square feet; 

U= utilization efficiency from Table 10; 

I = intensity of illumination in foot-candles; 

W = total watts required to produce the intensity 7; 

L = total lumens which must be produced by the lamps; 
l — lumens per watt for the particular lamp used. (See 
Tables 2 and 3); 

w = watts per square foot required to produce the intensity 
7. Then 


IXA 



( 1 ) 


_L_IXA 
l l XU' ' 


(3) 


Example 1 . Required the power necessary to produce 3 foot- 
candles uniform illumination in a room having an area of 1000 square 
feet. Ceilings and walls light. Prismatic reflectors and vacuum 
type, 100-watt lamps used. 


3 X1000 

- =5670 lumens, 

0.53 


Total lumens (L) = 


™ , 5670 

Total w’atts (IF) =—- 

10.30 


= 550 watts. 


Example 2. Required the power for Example 1 when gas-filled 
lamps and steel reflectors are used. Ceiling light, walls medium. 


3 X1000 
0.46 


Total lumens ( L ) = 


= 6521 lumens. 


6521 


Total watts (IF) =- 

12.6 


= 518 watts. 


Example 3. A room 40 by 20 feet is lighted with 24 vacuum-type 
tungsten lamps rated at 60 watts and equipped with translucent 
glass reflectors. The walls are a medium light buff, and the ceilings 
a very light buff. The lamps are evenly spaced. Calculate the 
average intensity on the working surface. 


Total power (IF) =24X60=1440 watts. 


From Table 11, the walls would be classed as “medium” and the 
ceiling as “light.” Hence U from Table 10 is 0.44. 


The intensity I = 


1440X0.44X9.8 


= 7.7 foot-candles. 


40X20 











94 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


Example 4 . A room 20 by 60 feet is lighted by three 300-watt 
(110-volt) gas-filled units with prismatic glass reflectors. The walls 
and ceilings are light. Calculate the intensity. Table 3 shows that 
each lamp produces 4600 lumens. 


I 


4600 X3 X0.60 
20 X60 


= 6.9 foot-candles. 


It is usually more convenient to determine the watts per square 
foot required to give a certain intensity. In example (1) 
this would be 548-1-1000=0.548 watt. By using values of this 
kind, the proper size of lamp can be more easily selected.* 
Using the factors from Table 10, the watts per square foot 
required to give an intensity of 1 foot-candle can be calculated. 
These values are given in Table 12, which is based on Mazda B 
lamps operating at 1.28 watts per m.s.cp. (1.0 watt per m.h.cp.) 
and producing 9.8 lumens per watt. Referring to Table 2, it 
will be seen that the lumens per watt vary somewhat with 
different sizes of lamps. Also if gas-filled lamps are used 
the values given in Table 12 must be multiplied by 0.64, 
which is an average value and corresponds to 300- and 400- 
watt lamps. For larger sizes the factor is somewhat less, 
and for smaller lamps it is greater. Table 12 can be used, 
however, to make the first determination, and then, if 
greater accuracy is required, the intensity can be calculated 
by means of the formulas given in this paragraph. Extreme 
accuracy is not required, however, since an allowance of from 
10 to 25 per cent must be made for depreciation due to dust. 
Referring to Tables 10 and 12, it will *be noted that the 
required watts per square foot are affected materially by 
the color of the walls and ceiling, particularly in the case of 
indirect systems. To assist in choosing the proper value, a 
color classification is given in Table 11. This shows the 
various shades of color corresponding to the designations 
used in Tables 10 and 12. 

Example 5. A room 40 by 20 feet is to be lighted with vacuum- 
type lamps using steel reflectors and a direct system. The ceiling is 
white and the walls are a light buff. Calculate the watts per 
square foot and the total power required to produce a uniform 
intensity of 4 foot-candles. Referring to Table 11, we find that the 
walls would be classified as “medium.” Using the column for light 
ceiling and medium walls in Table 12, we find that 0.222 watt per 

* The method of doing this is explained in paragraph 123. 



PAR. 121 ] 


POWER REQUIRED 


95 


square foot is required to produce an intensity of 1 foot-candle. 
Hence the power required is 

w =0.222 X4 =0.888 watt per square foot. 

or 

W =0.888 X40 X20 =710 watts. 

If gas-filled lamps were used the power would be 

0.64 X0.888 =0.57 watt per square foot. 

In all these examples no allowance has been made for a decrease of 
intensity due to dust on the light-units. 

In Table 13 is given the power required for the usual kinds of 
lighting service. This table can be employed to avoid the calcu¬ 
lations just described, and it is particularly useful for prelim- 
, inary calculations or for checking the results of a more careful 
determination. If the conditions of the problem are unusual, 
the power should be determined by the aid of Tables 8 or 9 
and Table 12, as outlined above. Extreme accuracy in these 
determinations is not required, however, as it is seldom possible 
to use the exact power calculated, because it is necessary to 
use standard size units and to space them to conform to the 
arrangement of windows, girders or columns. 

121. Other Lamps. The watts per square foot calculated as 
described are for uniform illumination with tungsten lamp 
units evenly spaced over the entire area considered. For 
flame-arc and mercury-vapor lamps, it is necessary to approxi¬ 
mate the required power by assuming a value of watts per 
square foot based upon similar installations. Approximate 
values for these lamps are given in Tables 14 and 16. 

122. Non-uniform Illumination. When local lighting is used 
alone or is combined with general lighting (uniform) the power 
required for this local lighting must be added to the power 
required for the uniform lighting. There is no simple method 
of calculating the required size of lamp to use for this local 
lighting, and reliance must be placed upon data obtained from 
installations similar in character. Tables 8 and 9 contain suc- 
gestions for the usual requirements. With localized general 
lighting, the power can be calculated only after the number and 
size of units have been determined. Here, also, the size cannot 
be easily calculated from the required foot-candles and experi¬ 
ence with similar installations is the best guide. 


96 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


* (7) Size of Light Unit 

123. General Lighting. With this method of lighting, it is 
customary to make the units all of the same size, and to so 
choose the size that the total power will be about equal to the 
total watts (IF) which must be used to produce the required 
illumination. The size of unit will therefore depend upon the 
number of units selected. In general, for direct systems, 
it is not satisfactory to employ only one or two units except 
in very small rooms. The size of the unit which it is best to 
use depends primarily upon the height of ceiling. For low 
ceilings, small sizes of units should be used, while for higher 
ceilings the larger units are most suitable. From this it follows 
that arc lamps are suitable only for high ceilings where they can 
be mounted well above the usual line of vision. In Table 15 
are given the sizes of lamps best suited for various ceiling 
heights. The exact size chosen will be settled when the spacing 
and power requirements are finally determined, but the size 
used should be checked by comparison with this table. For 
indirect and semi-indirect systems, the size is chosen from a 
knowledge of the required spacing, as will be explained in 
paragraph 125. 

124. Non-uniform Lighting. For local lighting the size of 
tungsten lamp used would depend upon the intensity of illu¬ 
mination desired. In general, however, a 50-watt lamp would 
be the largest size required, and in many cases, as for example 
in lighting sewing machines, a much smaller size may be used. 
For group lighting, the size would depend upon the height of 
ceiling, the same as for uniform illumination. 

(8) Location of Light Units 

125. The spacing of units depends principally upon the extent 
to which shadows must be eliminated and a good diffusion of 
the light secured. This is particularly true of direct systems. 
A wide spacing, besides making the illumination less uniform, 
also gives stronger shadows and requires the use of larger units. 
A very close spacing, while giving a more uniform illumination 
and eliminating shadows to a considerable extent, results in a 
more expensive installation, so that a balance must be reached 
by considering the requirements of each class of installation. 


PAR. 125 ] 


LOCATION OF LIGHT UNIT 


97 


It is apparent that a closer spacing would be required in a draft¬ 
ing room, where shadows should be practically eliminated, than 
in a factory where rough manufacturing is carried on. Tables 
1/ and 18 give desirable spacings for various classes of service 
for direct and indirect systems. For high ceilings wide spacings 
and large units are used. The tables give maximum distances 
and these should not in general be exceeded. It is also best not 
to use the widest spacing with the lower ceiling height. These 
tables apply to general lighting with uniformly spaced units. 



Fig. 54. —Method of Locating Light Units for Uniform Illumination. 

The total area to be lighted may be divided into squares (or 
rectangles which are nearly square) with a light unit located 
at the centre of each rectangle. The length of the sides of the 
rectangle is the spacing of the units. Fig. 54a shows the correct 
way in which to locate the units. They should not be located 
at the corners of the squares as shown in Fig. 54 b, because this 
places the outside rows too far away from the walls. The 
size of the rectangle which should be illuminated by one lamp 
depends upon the height at which the lamp can be mounted. 
This is of course affected by the height of the ceiling. If an 
attempt is made to use a large spacing with a low ceiling the 
light rays must be thrown out at wide angles and it is difficult 
















98 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


to secure uniform illumination. On the other hand, if a very 
small spacing is used, the cost of the installation will be too high. 
In general, therefore, close spacing and small-size units should 
be used for low ceilings and large-size units and wide spacing 
should be used for high ceilings where the lamps can be mounted 
at a considerable distance above the floor. The exact spacing 
and hence the size of the rectangle is fixed as soon as the watts 
per square foot and the most desirable size of unit are settled 
upon. Thus, if we require 0.5 watt per square foot to give the 
required intensity and after reference to Table 15 we decide 
that 100-watt units should be used, the number of units must 
be so chosen that there will be one for every 200 sq.ft, of area. 
Hence we can consider that each lamp illuminates a square 
which is \/20d = 14.13 ft. on a side. The lamps should there¬ 
fore be spaced 14.13 ft. apart and located at the centres of the 
squares as indicated in Fig. 54a. If 60-watt lamps were used 
each lamp could illuminate only 60^0.5 = 120 sq.ft, and the 
lamps must be spaced \/V20 = 10.95 ft. apart. To assist in 
determining the required spacing for various sizes of units, 
Fig. 55 is used. 

Example 1. Required the spacing to allow 1.0 watt per square 
foot in an office. 

From Fig. 55 we have: 

For 25 watt-units. 5.0-foot spacing 

40 6.4 

60 7.8 

100 . 10.0 


The spacing chosen must suit the requirements of Table 15, and 
should check with the values given in Tables 17 and 18. Thus, for 
a 14-ft. ceiling 100- or 150-watt units are desirable. Table 17 in¬ 
dicates that for direct systems the spacing should be from 9 to 14 
ft. for ceilings 12 to 16 ft. high. Hence a 100-watt lamp with a 
spacing of 10 ft. would be satisfactory. If 150-watt units are used, 
the spacing should be 12.2 ft., which would be undesirable from 
the standpoint of shadows. If 60-watt units are used, the spacing 
should be 7.8 ft., which is too close for an economical installation. 


The chart (Fig. 55) can of course be used for all styles of 
lamps, both arc and incandescent, 'provided, the required, watts per 
square foot are calculated for the particular type of lamp used. 

Example 2. Find the spacing for gas-filled lamps to give the 
same illumination as in Example 1. This would require 0.64 watt 






PAR. 126 ] 


LOCATION OF LIGHT UNIT 


99 


per square foot instead of 1.0 watt required for vacuum lamps. 
From Fig.' 55 we have: 

For 100 watt units. 12.6 ft. spacing 

75 . 10.8 

The 75-watt size could therefore be used instead of the 100-watt vacuum 

lamp. 



1h 


o3 


-Cl 


u 


tc 


o 

a 

a 

m 


»o 


<5 


126. Arrangement of Units. When the desirable spacing has 
been determined, as explained in the previous paragraph, the 
arrangement can be settled upon. To give uniform illumina¬ 
tion. the units should all be of the same size and should be 
* ^ 





















































































100 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


located in regular rows about as shown in Fig. 54a. If there are 
no columns in the room and the ceiling is not divided into 
bays by beams or girders, the arrangement shown in Fig. 59 
may be used. The lamps should be located in parallel rows 
with the spacing between rows and between lamps in the same 
row as nearly as possible equal to the value obtained from 
Fig. 55. The distance between the walls and the nearest row 
should be one-half the distance between the other rows, unless 
benches are located along the wall, when the first row should 
be about 12 to 18 in. nearer the wall than the edge of the 

bench. Sometimes a staggered 
arrangement of lamps (Fig. 56) 
is used. This may be necessary 
to obtain the required illumina¬ 
tion with the sizes of lamps 
available. In general, however, 
such an arrangement should be 
avoided, because of the irregular 
appearance of the lamps. When 
there is a line of columns running 
the length of the room or where 
the ceiling is divided into bays 
by means of deep girders or 
beams, each - bay should be con¬ 
sidered separately and the lights 
so spaced as to avoid trouble¬ 
some shadows. The arrangement 
in this case is shown in Fig. 58. 
The line of columns divides the 
room into sections and the units are located symmetrically 
with this row of columns. The beams, on a line with the 
columns divide the ceiling into bays (in this case 15 by 23 ft.), 
and the lamps must be located symmetrically in each of these 
bays in order to produce a good appearance. Where group 
lighting is employed, the units are so located as to give the proper 
intensity upon the machine. This would result in different 
spacings of the units, but usually they may be located in straight 
rows running the long way of the room, and this is desirable 
where possible, because of the appearance. 

127. Mounting Height. The height at which the unit should 
be mounted depends upon the system used. Where general 



Fig. 56. —Staggered Arrange¬ 
ment of Light Units. 








PAR. 127 ] 


MOUNTING HEIGHT 


101 


lighting is to be provided, the units must be mounted high 
enough to be out of the range of vision to avoid glare.* In 
general, the lamps should be at least 8 ft. above the floor and 
higher if possible. For ceilings from 11 to 16 ft. high the lamps 
should be about 10 ft. above the floor. For higher ceilings, 
cranes or other obstructions usually fix the minimum mount¬ 
ing height. If deep girders divide the ceiling into bays, the 
lamps should be located slightly below the bottom edge of the 
girders if possible. It might be considered that the height of 
the lamp above the floor would have an important effect upon 
the intensity of illumination which would be secured by the 
lamp, and this would be the case (according to the inverse 
square law)f if no reflectors were used. When, however, 
we use a reflector which directs the light in the proper direction, 
we can secure practically the same amount of light upon 
the working surface regardless of the height of the units. This 
is shown in Fig. 53. Accordingly, if we change the style of 
reflector when the mounting height changes, we can obtain 
the same illumination with the same expenditure of power. 
In other words, the values of watts per square foot required to 
produce a certain intensity as given in Table 12 can be used 
without correcting for different mounting heights. For group 
lighting, the same rules apply as regards the mounting height. 
For local lighting the units are mounted as close to the work 
as possible. With semi-indirect or indirect systems, the 
desirable spacings are given in Table 18, together with the hang¬ 
ing height of the fixture, or the distance between the fixture 
and the ceiling. With direct units, this distance is not im¬ 
portant, but for the other systems it is necessary to choose a 
suitable hanging height in order to properly distribute the light 
reflected from the ceiling. The hanging height varies some¬ 
what with different makes of fixtures, but the values given are 
representative of the ordinary units. 

Example 1. An office having a 12-ft. ceiling is to be provided with 
direct lighting and no desk lamps. Hence from Table 17 a mounting 
height of 9 to 12 ft. would be used with a spacing of 7 to 11 ft. In this 
case, a spacing of about 10 ft. with a mounting height of 9 or 10 ft. 
would be satisfactory. 

Example 2. For an indirect system, Table 18 indicates that for 
a 12-ft. ceiling the spacing should not exceed 18 ft. and the distance 
from the ceiling to the fixture should be from 2.5 to 3.0 ft. 

* See paragraph 74. t See paragraph 66. 


102 CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


(9) Selecting Reflector or Globe 

128. The general type of reflector, whether glass or metal, 
must be settled before the power required for illumination can 
be calculated as explained in paragraph 120. After the size 
of the unit has been selected, the proper size and shape of re¬ 
flector can be chosen to suit the particular lamp employed. 
Usually reflectors for direct lighting are suitable for use with 
only one or two sizes of tungsten lamps. The shape of the 
reflector, which determines the distribution of light, must be 
selected with proper regard to the arrangement of the units. 
It has already been pointed out* that if uniform illumination 
is to be secured, the style of reflector selected will depend upon 
the mounting height and spacing of the units. Thus wide 
spacing and low mounting height require a reflector giving an 
extensive distribution, while for closer spacings an intensive 
or a focussing distribution is required. Fig. 57 shows these 
relations for various reflectors used for direct lighting. The 
curves indicate the style of reflector required to give uniform 
illumination on the working plane, which is assumed to be 
2.5 ft. above the floor; but the mounting height is specified 
from the floor. If the work is at a different level than the one 
assumed, proper allowance must be made for the difference. 

Example 1 . For reflectors spaced 10 ft. apart the following styles 
should be used: 

Extensive for a height above the floor of about 7 ft. 6 in. 

Intensive for a height above the floor of about 10 ft. 

Focussing for a height above the floor of about 13 ft. 

Focussing for a height above the floor of about 16 ft. 

Example 2. If the reflectors must be mounted about 15 ft. above the 
floor, the following styles should be used: 

Extensive for a spacing of about 25 ft. 

Intensive for a spacing of about 15 ft. 

Focussing for a spacing of about 10 ft. 

Besides choosing the proper size of reflector for the lamp used 
and the proper shape to give the desired distribution of light 
for the spacing and mounting height, it is also necessary to 
choose the proper style of shade holder so as to centre the lamp 


* See paragraph 119. 


PAR. 128 ] 


SELECTING REFLECTOR 


103 


in the reflector. If this is not done, the distribution of light 
will not be as designed, and uniform illumination will not be 



secured. For indirect and semi-indirect systems, the size 
of bowl used would depend principally upon the ceiling height, 
which would fix the size of unit in accordance with Table 15. 


















































104 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


Examples 


129. Office. Size 46 by 45 ft. with a 14-ft. ceiling. The 
columns and girders form six bays, each 15 by 23 ft. The 
lighting would be classed as commercial and good diffusion and 
pleasing appearance are essential. For these reasons a semi- 
indirect system will be used. Tungsten lamps would of course 
be employed. Referring to Table 8, it will be seen that large 
offices require from 4 to 8 foot-candles when no desk lamps 
are used. Because the conditions are average we will assume 
6 foot-candles. Allowing 20 per cent depreciation due to dust, 
because semi-indirect bowls gather dust faster than direct 
systems, the intensity with clean units should be 1.20X6=7.2 
foot-candles. The ceilings are ivory white and the walls a very 
light buff, so both would be classed as “ light .” (Table 11.) 
Table 12 shows that for vacuum-type lamps, 0.291 watt per 
square foot is required to produce 1 foot-candle. The required 
power is then 0.291X7.2=2.09 watts per square foot. The 
area of the room is 46X45=2070 sq.ft. Since the values in 
Tables 10 and 12 are based on areas of 200 to 1000 sq.ft., the 
power required would be somewhat less than the value calcu¬ 
lated. With light walls, however, it is not well to make any 
change. Since the room is divided into bays 15 by 23 ft., 
we could have one, two, or four lamps per bay. Referring to 
Table 18, it will be seen that for a 14-ft. ceiling the spacing should 
not exceed 24 ft. If we used one lamp per bay, the spacing 
would be 23 by 15 ft. While this might be used, the consider¬ 
able difference between the lengths of the two sides of the 
rectangle illuminated by one lamp would make it preferable 
to use two units per bay, giving a spacing of 15 by 11.5 ft. 
The average spacing is then 13.25 ft. Reference to Fig. 55 
indicates that about 360 watts per unit are required. If we 
used three 100-watt lamps per unit, we would have; 


W = 3X2X6X100 = 3600 watts, 


w = 


3600 

2070 


= 1.74 watts per square foot, 


which is somewhat less than required. The intensity can bo 
calculated from formula (4): 


I 


3600X0.35X10.30 


= 6.3 foot-candles. 


2070 




PAR. 129 ] 


EXAMPLES 


105 


This is slightly low. If four 100-watt lamps were used, we would 
have 4800 watts total or 2.32 watts per square foot, and this 
would give an intensity: 


4800X0.35X10.30 

2070 


= 8.37 foot-candles. 




Fig. 58.—Lighting Plan for an Office. 


This is about 16 per cent too high, but in general this value 
would be chosen rather than the lower value in order to be on 
the safe side. The arrangement of units is shown in Fig. 58. 
According to Table 18, the hanging height should be from 



















































































106 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


3.5 to 4 ft. In this case, since the spacing is considerably 
less than the maximum, a height of 3.5 ft. would give the best 
appearance. The lamps would be mounted horizontally inside 
a suitable bowl. If gas-filled lamps are used, the power would 
be 0.291 X0.64=0.186 watts per square foot. To produce 
7.2 foot-candles, the power is 0.186X7.2 = 1.34 watts per 
square foot. Two units per bay would be used as before, 
giving an average spacing of 13.25 ft. From Fig. 55 we find 
that about 240 watts per unit are required. The nearest stand¬ 
ard size is 200 watts, which gives a total of 2400 watts or 1.16 
watts per square foot. The intensity is 


I 


2400X0.35X14 
2070 " 


= 5.7 foot-candles. 


If 300-watt lamps are used, the total power is 3600 watts, giving 
1.74 watts per square foot and an intensity 


3600X0.35X15.3 
~ 2070 


= 9.3 foot-candles. 


It is apparent that neither of these sizes fits the case exactly. 
If it is assumed that the lamps will be cleaned very frequently 
and the character of the work is not too exacting, 200-watt 
lamps would be satisfactory. If there is a chance of dust 
accumulating, the 300-watt lamps would be better. A single 
lamp would be used in each unit because gas-fillecl lamps operate 
better in a vertical position. 

130. Store. Dimensions: 25 ft. wide, 60 ft. long, 13-ft. 
ceiling. Dry goods store displaying both light and dark goods. 
The walls are covered with shelves and therefore should be 
classed as “ dark.” The ceiling is “ light.” General illumina¬ 
tion by the direct system will be employed, to reduce the first 
cost of installation as much as possible. Vacuum-type tungsten 
lamps with velvet finished, prismatic reflectors are suitable. 
Since the store is located in a small city, a moderate intensity 
is satisfactory. Table 8 indicates that from 4 to 7 foot-candles 
are required. For this store, 4 foot-candles would be satis¬ 
factory. Allowing 10 per cent for depreciation, we should 
start with 1.10X4=4.4 foot-candles. The power required to 
produce 1 foot-candle is 0.227 watt per square foot (Table 12). 
The required power is 0.227X4.4 = 1.0 watt per square foot. 




PAR. 130 ] 


EXAMPLES 


107 


The total area is 60X25 = 1500 square feet. Table 17 indi¬ 
cates that the desirable spacing for stores with ceilings 11 to 
15 ft. high is 10 to 16 ft. If one row of lamps is used, it would 
not be possible to get good lighting on the counters. With 
two rows, the spacing between rows would be 12.5 ft. With 
5 lamps per row the spacing the other way would be 12 ft. 
Fig. 55 indicates that for 1 watt per square foot 150 watt 
units must be used. These are not made for 110-volt service 
(see Table 2). If 6 lamps are used per row, the spacing becomes 
10 by 12.5 ft., an average of 11.25 ft. To give 1 watt per square 
foot would require (Fig. 55) about 125 watts per unit. Using 
100-watt lamps would give 1200 watts total power or 0.8 
watt per square foot. The intensity would be 

7 =--=3.7 toot-candles. 


This is too low. With gas-filled lamps and the same kind of 
reflectors, the power to produce 1 foot-candle is 0.213X0.64 
= 0.136 watt per square foot or 0.136X4.4=0.6 watt per square 
foot to produce 4.4 foot-candles. With 5 lamps per row, 
giving a spacing of 12.5 by 12 ft., a 100-watt unit could be 
used. This gives 1000 watts total or 0.67 watt per square foot. 
The intensity is 


/ 


1000X0.48X12.6 

1500 


= 4.0 foot-candles, 


slightly low. Using 6 lamps per row gives 1200 watts total, 
0.8 watt per square foot and an intensity 

j 1200X0.48X12.6 . Q , , „ 

I =- - —-—— =4.8 root-candles. 

1500 

While this is slightly high, it is a better value than the other, 
as it allows more for depreciation due to dust. Since the area 
is somewhat more than 1000 sq.ft., according to the note under 
Table 10, the factor 0.48 might be increased somewhat. This 
applies to rooms which are nearly square, in which case the light 
from other bays helps to raise the illumination in the central 
portion of the room. For long, narrow rooms, such as we have 
here, it would not be safe to increase this quantity. The arrange- 





108 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


"s; 


"S" 


j±> 

^CO 


ment is shown in Fig. 59. The units should be mounted as 
high as possible because of their brightness. Assuming a height 

of 12 ft. above the floor, Fig. 
57 shows that for an average 
spacing of 11.25 ft. intensive 
reflectors should be used. 

131. Machine Shop. The 
shop is 128 ft. long by 44 ft. 
wide and is divided into 16 bays 
each 16 by 22 ft. The ceiling 
is 14 ft. high. Floor area 44 X 
128 = 5632 sq.ft. Rather close 
work is carried on. Direct 
lighting with tungsten lamps, 
using steel reflectors, is most 
suitable. From Table 9, we find 
that for machine tools and fine 
work from 5 to 8 foot-candles 
are required. Assume 6 and 
allow 25 per cent for dust. The 
intensity with clean lamps should 
be 1.25X6 = 7.5 foot-candles. 
The ceiling is a very light buff 
and the walls a medium light 
buff. Hence the classification 
is “ light ” and “ medium ” 
(Table 11). From Table 12, for 
vacuum-type lamps 0.222 watt 
per square foot is required for 1 
foot-candle, or 7.5X0.222 = 1.68 
watts per square foot for the 
required illumination. Either 
one, two or four units could be 
used in each bay. With four 
units the average spacing is 9.5 
ft. and 150-watt units would 
be required (Fig. 55). Vacuum- 
type lamps are not commonly 
used in this size. With gas-filled lamps the watts per square 
foot would become 0.222X0.64 = 0.142 watt per square foot 
for 1 foot-candle or 7.5X0.142 = 1.07 watts per square foot for 



Fig. 59. —Lighting Plan for a 
Store. 

















































PAR. 131 ] 


EXAMPLES 


109 


the required illumination. With one unit per bay, the spacing 
would be 16 by 22 ft. and 400-watt units would be required. 
With two per bay, the spacing is 16 by 11 ft. and 200-watt units 
are necessary. With 4 per bay, 100-watt units with a spacing 
of 8 by 11 ft. would be required. Table 17 indicates that 


r ir ' r 


r ..it " 

i 7 

r 

r 

- 

e 

r 

f 

' 

4 




^aa^ 




for close work for ceilings 11 to 15 ft. high, spacings of 8 to 
13 ft. should be used. Four 100-watt units would there¬ 
fore give the best arrangement. The total power would be 
4X16X100=6400 watts, giving 1.14 watts per square foot. 
The intensity would be 


/ = 


6400X0.46X12.6 

5632 


6.6 foot-candles. 






















































































110 


CALCULATING INTERIOR ILLUMINATION [CHAP. 7 


This is lower than the required value of 7.5 foot-candles. Since 
this is a large room the utilization efficiency would be somewhat 
higher than 0.46. (See note for Table 10.) We have already 
made allowance for dust, and therefore, unless there were many 
obstructions such as belts or pipes, the illumination would be 
sufficient. The arrangement is shown in Fig. 60. The working 
plane in this case would be more than 30 in. above the floor 
and can be assumed as 4 ft. Hence the mounting height should 
be 18 in. more than the value given in Fig. 57. If an inten¬ 
sive style of reflector is used with an average spacing of 9.5 ft. 
it should be mounted 10 + 1.5 = 11.5 ft. above the floor in this 
case. Enamel steel reflectors would be the best kind to use. 


CHAPTER 8 


OUTDOOR LIGHTING 

Street Lighting 

132. Street lighting is employed to aid traffic movements, 
to assist in policing the streets and in some cases to illuminate 
building exteriors and attract trade. 

133. Systems Used. As a rule, street lighting is furnished 
by the series system because the cost of circuits is less.* Alter¬ 
nating current is generally used. Direct current is necessary 
where metallic-electrode arc lamps are used. For a.c. circuits, 
6.6, and 7.5 and sometimes 10 amperes are used; for d.c., 
4.5 and 6.6 amperes are standard. The voltages for series 
circuits are high. With arc circuits 50 or 75 lamps are frequently 
operated on a single circuit. Allowing 84 volts per lamp,f 
the total voltage becomes 6300. Series incandescent circuits 
are somewhat lower in voltage. The power factor of a metallic- 
electrode arc-lighting system, including the regulating trans¬ 
former, is about 0.65. For incandescent-lamp systems, with 
regulating transformers, it is about 0.83. At the present 
time, series circuits are almost always supplied from a con¬ 
stant-potential, a.c. source. Arc-light generators are seldom 
used. The common arrangement is a constant-current trans¬ 
former, which keeps the current in the circuits constant regard¬ 
less of the number of lamps in circuit. These transformers 
are built for different numbers of lamps, and as a rule operate 
a single circuit. In the larger capacities up to 100 lights, two 
circuits are supplied from a single transformer. When direct 
current must be supplied, a Cooper Hewitt mercury-arc recti¬ 
fier is connected to the constant-current side of the regulating 
transformer. For tungsten lighting circuits, regulating trans¬ 
formers are used in many cases. Another arrangement is the 

* See paragraph 212. 

t Including a line drop of 4 volts per lamp. 

Ill 


112 


OUTDOOR LIGHTING 


[CHAP. 8 


Primary Windings 

Taps 

/ yW A AVWVW|^ p SeC a°Dd d Tap9 WindiDg 


13 13 

■TX - CX 


U U U U 


Reactance Coils Mounted in Hoods 

1 3 13 1 3 31 1 3 

-xr—xx— Tx —Ti—xp— 

IA w IA vs (eTBroken Lamp, 

Coil Operating 

Fig. 61. —Diagram of Adjuster-socket, 
Series System. 


adjuster-socket system (Fig 61). A transformer is provided 
to insulate the series circuit from the supply feeder, and taps 
are employed to allow adjustment for different numbers of lamps. 
In general, the adjuster-socket system is best for small or 
medium-size systems. One advantage of this system is that 

the transformer re- 
sup piy i. MaTna quires no attention and 

can be mounted on a 
pole. A constant-po¬ 
tential system is some- 
1 3 13 times used for street 

lighting in the sections 
of large cities which have 
Edison mains. The 
lamps, either arc or in¬ 
candescent, are turned 
on by hand at each 
pole, or are controlled 
by time switches. 

134. Lighting Units.* For arc-lighting systems, the metallic- 

electrode lamp is standard. Flame-arc lamps are used to some 
extent. Enclosed and 
open arcs are obsolete 
and are being rapidly 
discarded. For in- 
candescent-ligh ti n g 
systems, gas-filled 
tungsten lamps are 
generally used. They 
are made in various 
sizes f to suit the dif¬ 
ferent capacity cir¬ 
cuits and are fre¬ 
quently installed in 
the same circuit with Fig. 62.- 
arc lamps. There 
seems to be a ten¬ 
dency, at present, to replace arc lamps with tungsten lamps, 



-Street Lighting Unit, Small-size 
Lamps. 


* A description of arc and incandescent lamps: used for series circuits is 
giVnn in Chapters 2 and 3. 
t See paragraph 31. 

























































PAR. 134] 


STREET LIGHTING 


113 


in spite of the fact that both flame- and metallic electrode 
arcs are more efficient. The style of fixture used with 
incandescent lamps depends upon the service. For small 
towns and the thinly settled districts of larger places, bare lamps 
with enamel-steel reflectors are satisfactory (Fig. 62). For 
larger lamps, which are used in residential or business sections 
where appearance is important, the form of fixture shown in 
Fig. 63 is used. When used 
on series systems, a trans¬ 
former is generally mounted 
inside the casing to allow 
the use of 15- or 20-ampere 
lamps on standard series 
circuits. For business 

i 

streets and boulevards, 
ornamental standards are 
frequently used (Fig. 64). 

The single-light standards 
are equipped with a large 
tungsten lamp or a mag¬ 
netite or flame-arc lamp. 

Multiple-light standards 
use gas-filled lamps. For 
series circuits, some form 
of lamp cutout must be pro¬ 
vided to keep the circuit 
closed under all conditions. 

For arc lamps this is com¬ 
bined with the lamp mech¬ 
anism and for incandes¬ 
cent lamps it is contained 
in the lamp socket and 
system, a reactance coil is shunted across each of the 
lamps (Fig. 61). This is so designed that when the lamp is 
operating very little current flows through the coil. When the 
lamp burns out, the entire current flows through the coil. 
The voltage drop of the coil is such that the amount of current 
is not affected greatly until about 20 per cent of the lamps are 
out. When the regulator system is used for incandescent 
lighting, a film cutout is employed. A porcelain receptacle 
containing two spring clips is mounted in the lamp hood. These 



Fig. 63.—Typical Outdoor Unit for 
Gas-filled Tungsten Lamp. 


hood. With the adjuster-socket 


























114 


OUTDOOR LIGHTING 


[chap. 8 


clips are connected to the circuit and are in contact when the 
lamp is removed. The lamp is provided with a porcelain socket 
having corresponding spring clips which are separated by a thin 
insulating film. When the socket is plugged into the receptacle, 
the spring clips on the latter are separated and the current 

flows through the lamp. If the 
lamp burns out, the voltage at the 
lamp terminals rises and punctures 
the film, thus short-circuiting the 
lamp. The film punctures at about 
400 volts. 

135. The intensity of illumina¬ 
tion required depends upon the use 
which is made of the light. In any 
case the required intensity is much 
less than is necessary for interior 
illumination. For thinly-settled 
districts, the illumination is usually 
sufficient only to define the road. 
Residence districts require a some¬ 
what higher intensity to enable 
obstructions and irregularities in 
the pavement to be seen, and to 
assist in policing the streets. 
Business districts require a still 
higher intensity because the traffic 
is heavier and also for the adver¬ 
tising effect. Table 19 is repre¬ 
sentative of good practice. 

136. Arrangement of Units. In 
general, no attempt is made to 
obtain uniformity of illumination, 
although when the units are closely 
spaced for “ white way” lighting 

this condition is approximately fulfilled. In residence streets, 
where there are a number of lamps closely spaced, better results 
as regards visibility are secured if the lamps are all placed on one 
side of the street. As regards size of unit, it may be said that 
an installation of large lamps, widely spaced, costs less and 
directs the light onto the street at a better angle than a large 
number of small units. Sometimes small units must be used 



ing Standards. 

a. Single light unit for metallic- 
electrode or flame-arc lamps or 
large tungsten lamps, b. Multiple 
light unit for tungsten lamps. 











































PAR. 137 ] 


YARD LIGHTING 


115 


because of the presence of trees. The spacing varies with the 
character of the street. For thinly settled districts, the spac¬ 
ing is great and frequently irregular. In such cases lamps are 
placed at street intersections and at the outer edge of curves 
in the road. The actual distance between lamps may vary 
from 500 to 1000 ft. For residence streets where small 
size units are used, 100-ft. spacing is satisfactory. If large 
units are used, they would be placed at each street intersec¬ 
tion. For business districts the spacing varies from 100 to 
200 ft., depending upon the size of unit and the importance of 
the street. The mounting height depends upon the size of 
unit and the presence of trees, etc. An increased height in¬ 
creases the cost of the installation and reduces the amount of 
light which reaches the street surface. The height should be 
from 15 to 18 ft. for small units and from 20 to 25 ft. for larger 
sizes. 

Yard Lighting 

137. Railway Yards. The lighting of the freight classifica¬ 
tion yards of a railroad must be sufficient to allow work to be 
carried on safely and accurately at night. The units employed 
for this purpose include flame-arcs, quartz lamps, gas-filled 
tungsten lamps and arc search-lights. The tracks in the yard 
are placed so closely together that as a rule it is impossible 
to locate poles between the tracks. The light units are there¬ 
fore placed at the sides of the yard. Flame-arcs and quartz 
lamps should be mounted high enough to be out of the range 
of vision of the car riders. This requires that they shall be 
from 40 to 75 ft. above the ground. Gas-filled tungsten lamps 
with enameled steel reflectors can also be mounted in a similar 
manner. Another arrangement uses tungsten lamps with angle 
reflectors pointed away from the entrance to the yard. By 
this means, glare is eliminated and the lamps can be mounted 
lower, for example 30 to 40 ft. The units are spaced from 
300 to 600 ft. apart. The intensity should average from 
0.27 to 0.43 foot-candle and should not be less than 0.02 
foot-candle.* Storage and loading yards do not need as high 
an intensity as this. 

138. Factory Yards. Flame-arcs, quartz lamps or large 
gas-filled tungsten lamps are suitable for factory yards. 

* National Electric Light Association. Report of Yard Lighting Committee, 

1916. 


116 


OUTDOOR LIGHTING 


[chap. 8 


Tungsten lamps are most commonly employed in new installa¬ 
tions. The unit shown in Fig. 63 is very satisfactory. Unless 
there are high obstructions in the 3 ^ard, a mounting height 
of from 20 to 30 ft. should be used. The spacing is generally 
irregular, because it depends so much upon the arrangement 
of the buildings. The lamps would be mounted at intersec¬ 
tions of important roadways, with additional lamps between, 
if the distances are too great. For flame-arcs or quartz lamps, 
a spacing of from 300 to 600 ft. is satisfactory. Using 300- or 
400-watt tungsten lamps, a spacing of about 300 ft. would be 
required. In some cases, a 200-watt lamp, with a closer spac¬ 
ing, can be used to advantage. 

139. Tennis courts have been very successfully lighted by 
large tungsten lamps equipped with enameled-steel reflectors. 
With the side-lighting system six units are used on each side of 
the court. These consist of 400-watt gas-filled lamps equipped 
with 45° angle reflectors. The units are generally hung from 
cables run parallel to the side lines and about 2 ft. outside 
the court. The units are hung about 18 ft. high and spaced 
15.5 ft. apart. With the overhead-lighting system four 1000- 
watt units and bowl-shape, enameled-steel reflectors with 
extension skirts are used for a single court. The extension 
skirts shield the lamps and prevent glare. The units are hung 
from a cable in a single row, the long way of the court and over 
the centre line. They are hung about 30 ft. high and are 
spaced about 28 ft. apart. Where there are several courts 
side by side, four 750-watt units are hung in a row between the 
courts. The mounting height and spacing are the same as when 
mounted over the centre line of the court. The side lighting 
system costs about 50 per cent more than the overhead system, 
but the former seems to be preferred by players. 

Electric Signs 

140. Illuminated Signs. Tungsten lamps are used generally 
for electric signs. Three different voltages are used: 12* 
volts of 2.5 and 5-watt capacity; 60* volts of 5-watt capacity; 
and 120* volts of 7.5- and 10-watt capacity. Where the signs 
are placed close to the streets, frosted lamps are used. Tung¬ 
sten lamps are better than carbon lamps because they give a 

* Lamps with slightly lower or higher voltages can be obtained. 


PAR. 140 ] 


ELECTRIC SIGNS 


117 


more brilliant appearance to the sign and they also take less 
power. 1 or flashing signs, they have the further advantage 
that they reach full candlepower very quickly (about one- 
tenth of a second) and so can be used for very rapid flashing 
effects which would be impossible with carbon lamps. When 
direct current is used, there are several different methods of 
wiring the lamps (Fig. 65). The multiple arrangement (a) 
is the simplest but can be used only for 120-volt lamps. When 
60-volt lamps are used either (6) or (c) can be used. For 12- 
volt lamps, an arrangement similar to (6) or (c) would be used. 
For (6) there would be 10 lamps in series in each group and in 
(c) the total number of lamps in the sign would be divided into 


o - 
o 




o 

o - 

(jxjxj) <i>0<?<H 




J/ * 

Fig. 65. —Connections for D.C. Signs. 


10 groups and the lamps in each group placed in multiple. 
Arrangement (c) is better than (6) because if one lamp burns out 
none of the others are extinguished, whereas for (6) all the lamps 
in the same series are affected. It is very important to have the 
same number of lamps in each multiple group for arrangement 
(c), otherwise the voltages across the different groups would not 
be the same and some lamps would burn brighter than others. 
It is also important that all the lamps shall have the same 
rated voltage and watts for the same reason. If a lamp burns 
out, the voltage increases across the lamps in multiple with 
the burned-out lamp and decreases across the others. The 
amount of change depends upon the number of lamps in one 
multiple, a small number of lamps giving a large change in 
voltage. The amount of voltage increase on the remainder 
of lamps in a multiple having one or more burned-out lamps 









































OUTDOOR LIGHTING 


118 


[chap. 8 


i 

is given in the following tabulation. This is for an arrangement 
of 10 multiples in series (12-volt lamps). 



Per Cent Voltage Increase. 

20 Lamps in 

Each Multiple. 

10 Lamps in 

Each Multiple. 

1 lamp burned out. 

9 

18 

2 lamps burned out. 

18 

40 

3 lamps burned out. 

28 

70 

4 lamps burned out. 

40 



In general, it may be said that with 10 or more lamps in each 
group 1 lamp can burn out without causing the others to burn 



out within a reasonable time. When 60-volt lamps are used with 
connection (c) there is a possible danger due to the short cir¬ 
cuiting of one lamp. This would throw double voltage on the 
other group and would cause the lamps to burn out in a very 
short time, probably in less than a half hour. By close fusing, 
the lamps can be protected. It is necessary to select fuses which 
will carry the normal load but will melt when the current is 
about 50 per cent above normal. For this reason, the fuses 



































PAR. 141 ] 


ELECTRIC SIGNS 


119 


should be operated normally somewhat above their rating. 
When signs are used on alternating current, the lamps should 
always be arranged on the multiple system, using transformers 
when low-voltage lamps are required (Fig. 66). Special sign¬ 
lighting transformers having capacities from 50 to 2000 watts 
are made for this purpose. They are designed to give 12 volts 
on the secondary with 120 volts on the primary. The trans¬ 
former should be mounted as close as possible to the lamps 
to reduce the length of the low-voltage circuit, since it is neces¬ 
sary to keep the drop down to a very low value. For this reason, 
on large signs, it is best to use several small transformers rather 



Fig. 67. —Connections for Low-voltage, Multiple Sign Lamps. 

Taking the maximum voltage difference between lamps as 1 for (a), the other 
connections give differences as follows: (5) 0.25; (c) 0.25; ( d ) 0.06. 


than one large unit. The method of feeding low-voltage mul¬ 
tiples is important (see Fig. 67). System (a) gives the greatest 
difference in voltage between lamps. This is not important 
for 110-volt wiring, but for low-voltage systems (12 volts) 
the increased current may cause a large voltage difference. 
System (d) is best for low-voltage work. The maximum 
voltage difference should not exceed about 4 per cent of the 
lamp voltage. For such low-voltage systems, the feeders 
between the transformers and the lamps must be large enough 
to keep the drop down to about 2 per cent of the lamp voltage; 
in other words, the drop should be about 0.25 volt. 

141. Sign Flashers. Flashing signs are more satisfactory 
than fixed signs because they attract more attention. Thermo¬ 
flashers (Fig. 68a) have an interrupter which is operated by 
the heating effect of the lamp current. In the type shown, 
the lower arm is wound with a resistance wire which is connected 























120 


OUTDOOR LIGHTING 


[CHAP. 8 


to the two terminals. When the arm is cold, the contact 
is open and all the current for the lamps flows through the resist¬ 
ance. This reduces the current to the point where the lamps 
barely glow. The resistance heats the arm upon which it is 
wound, causing it to expand and the contact to close. This 
short-circuits the resistance and the lamp lights. As soon as 
the arm cools down the contact is opened and the lamp goes 
out. This process is repeated at frequent intervals. Flashers 



Lamp Circuits Slow Flashing 


Lamp Circuits RnpidJJlashing 



Fig. 68.—Sign Flashers. 

a. Thermo-flasher; 165 watts, 110 volts, b. Diagram of thermo-flasher, 
c. Motor flasher having four lamp circuits (3 amperes each), for high-speed 
flashing effects such as revolving borders and two circuits (10 amperes each), 
for slow effects, throwing the circuits on alternately. (Betts & Betts.) 


of this type are made in various capacities up to 330 watts to 
suit different numbers of lamps. They will work well only at 
approximately rated load. The larger sizes have spark elimi¬ 
nators consisting of condensers shunted across the contacts. 
A somewhat similar type with the contacts in a vacuum is made 
in capacities up to 12 amperes single pole and 25 amperes double 
pole. Thermo-flashers are made for operation on direct and 
alternating current. The disadvantage is that the rapidity 
of the flashes cannot be controlled accurately and therefore 





















































PAR. 141 ] 


SIGN FLASHERS 


121 


they can be used only for simple flashing effects. Magnetic 
flashers employ a switch operated by an electro-magnet. The 
time on and “ off ’ is controlled by a dash pot. These 
are used for the same class of service as the thermo-flashers 
but are not as satisfactory. Motor-operated flashers (Fig. 
68c) are used for large signs and complicated effects where the 



Fig. 69.—Diagrams for Motor-flashers. 

(a), ( b ) and (c) are for slow-speed “ on and off ” flashers; ( d ) and ( e ) use 
high-speed flashers. (Betts & Betts.) 


different lamps must be turned on in a definite order. The 
various lamp circuits are closed by brushes which strike contact 
sectors mounted on a motor-driven shaft. Generally several 
of these sectors (controlling different groups of lamps) are 
mounted together and supplied with current through a contact 
wheel and brush. The sectors are provided with sections which 
can be changed in different ways to adjust each circuit indepen- 



































































































































































122 


OUTDOOR LIGHTING 


[chap. 8 


dently. For 110-volt circuits the Code allows a maximum of 
30 amperes for a single-pole switch. In the type of flasher 
shown, it is customary to use double-pole switches above 20 
amperes. For 220 volts, the capacity is about 50 per cent 
less. For low-voltage work much larger currents can be 
carried. When a.c. signs are used with transformers and 
low-voltage lamps, the flasher is connected in the secondary 
citcuit, except for very large signs, where a separate trans¬ 




it 


Fig. 70.—Flood Lighting. 

a. Lighting bill board with angle reflectors, b. Flood lighting projector. 
Used to illuminate large signs and building exteriors, from a distance. In (a) 
the distance A should be about 0.5 B, except when i? is less than 9 ft., when A 
should be about 0.75 B. Distance C should be about 1 ft. Spacing between 
lamps should be about 6 ft. for signs up to 10 ft. high (B); above this, spacing 
should be 0.5 B. 


former could be used for each group of lamps. Diagrams for 
flashing signs are given in Fig. 69. In diagram (a), all the lamps 
are thrown on and off together, but a double-pole switch is 
used because of the large load. When single-pole flashers are 
connected as in (c), the two sides should be balanced as nearly 
as possible. 

142. Flood lighting for bill boards and painted signs is now 
commonly used. Tungsten lamps mounted in angle-type 
enameled steel reflectors are installed in front of and slightly 






























PAR. 142 ] 


FLOOD LIGHTING 


123 


above the top edge of the bill board. The size of lamp varies 
with the size of the sign. For a brilliant illumination of 10 
to 12 foot-candles about 2.5 watts per square foot of surface 
should be allowed. For moderate illumination of 5 to 7 foot- 
candles about 1.75 watts per square foot are needed. The 
arrangement is shown in Fig. 70. For the ordinary sign from 
9 to 12 ft. high, 200 or 100-watt, gas-filled tungsten lamps are 
commonly used. For large signs, which cannot be conveniently 
lighted as shown in Fig. 70a, flood-lighting projectors are used. 
A special form of tungsten lamp with a concentrating mirror 
reflector is employed so as to produce a strong beam of light. 
The projector can be located on a pole or the roof of a building 
at a considerable distance from the sign. Projectors are also 
used for lighting excavations for night work, for trap shooting, 
etc. 


PART II.—ELECTRIC POWER SYSTEMS 


CHAPTER 9 

MOTORS FOR INDUSTRIAL PURPOSES 

143. Advantages of the Electric Drive. In modern shop 

installations, the drive is almost invariably electrical. The old, 
mechanical drive, where the engine is connected to the machines 
by long lines of shafting, with numerous jackshafts and belts, 
is seldom adopted now for a new installation, particularly 
for large plants. Some of the advantages of the electric drive 
are: * (a) An increase in production. (6) A greater flexibility 
of arrangement of machines, (c) A clear headroom is provided. 
This permits the free use of overhead cranes, and results in 
better illumination and greater freedom from accidents. ( d ) 
Because the power can be transmitted electrically for con¬ 
siderable distances with small loss, the power plant can be 
centralized where the power can be generated most efficiently. 
Tests of shafting drives indicate that the loss varies from 25 to 
75 per cent and in most cases is about 50 per cent of the power 
generated, f Furthermore, the loss is nearly constant regard¬ 
less of the load. With the electric drive the loss depends upon 
the amount of power transmitted, and the total loss, including 
that in generators, feeders and motors, is only from 20 to 30 
per cent, (e) The electric service is more reliable, since a 
breakdown usually puts out of commission only a small part of 
the motors. (/) It is possible to operate sections of the factory 
at high efficiency when the other sections are shut down, whereas, 
without the electric drive, possibly all the shafting in a large 
mill might have to be operated for the purpose of driving one 

* See paragraph 187. 

t A. F. Strouse, Electric Journal , 1912, p. 209. 

124 



PAR. 144 ] 


ELECTRIC DRIVES 


125 


or two machines. ( g ) Speed control is easily secured by means 
of the electric motor. Instead of speed changes in large steps 
by changing gears or shifting belts, a very large number of 
small speed changes can be easily obtained. This results in 
keeping the operating speed closer to its economical limit and 
thus increases the production. ( h ) A study of machine per¬ 
formance can easily be made by introducing meters in the 
motor circuit. By this means, an accurate record may be 
obtained of the power requirements for a particular operation 
and any excessive demand for power due to the faulty working 
of the machine is easily detected. 

144. Systems of Power Supply. Of the two systems of power 
supply,* the multiple or constant-potential system is the only 
one used in this country for the operation of motors. At one 
time the series system was used for this purpose, but the advan¬ 
tages of the multiple system, due to the lower voltages on the 
motors and the entire independence of each machine, makes 
it the only practical system for general power service. No 
description will therefore be given of motors or their accessories 
for use on series systems. 

145. Classes and Types of Motors. Motors for industrial 
purposes are divided into two classes, direct current and alter¬ 
nating current according to the power supply used. The 
d.c. motors comprise three general types: series, shunt, and com¬ 
pound motors, depending upon the type of field winding used. 
The a.c. motors are also divided into three general types: 
induction, synchronous and commutator motors. The a.c. 
motors are further subdivided into single-phase and polyphase 
motors, the latter including both two-phase and three-phase 
motors. Electric motors are also classified according to their 
performance. With constant-speed motors, the speed is prac¬ 
tically constant regardless of the load (for example shunt 
motors) the maximum variation between no load and full 
load not exceeding about 20 per cent. In the case of varying 
speed motors such as series motors, the speed decreases greatly 
with increase in load. Adjustable-speed motors are so con¬ 
structed that the speed can be adjusted gradually over a wide 
range, but when once adjusted the speed remains practically 
constant regardless of the load. Examples of this type are the 
specially designed shunt or compound motors. 

* See Chapter 12 for description. 


126 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


Direct-current Motors 

146. Series Motors. In this type of motor the armature 
and field windings are connected in series, so that the entire 
armature current passes through the field circuit. The field 
is therefore wound with a small number of turns of large wire, 


To Line 



Fig. 71. —Diagram of Connections for a Series Motor. 


which will safely carry the armature current and will give a 
small drop in the field winding (Fig. 71). Since the current 
in field and armature is the same, the strength of the field in¬ 
creases as the armature current is increased due to an increased 
load. Hence series motors can start heavy loads without draw- 


Speed regulator 



Fig. 72.—Connections for Speed Adjustment of a Series Motor. 


ing an excessive amount of current from the line. The series 
motor is of the varying-speed type, since the speed rapidly in¬ 
creases as the load is decreased. If the load is entirely thrown 
off, a dangerously high speed may result, and therefore series 
motors should never be belted to the load, but should be either 
direct connected or equipped with a gear or chain drive. 





























I 


PAR. 147 ] SHUNT MOTORS 127 

Speed adjustment may be accomplished by means of a resistance 
in series with the motor (Fig. 72). This reduces the voltage 
on the motor and thereby reduces the speed. There is con¬ 
siderable loss in the rheostat, but since a series motor requiring 
speed adjustment is used only intermittently, this does not 
result in a large loss in power. This rheostat would be com¬ 
bined with the starting rheostat.* The direction of rotation, 
may be changed by reversing either the connections of the 
field or armature, but not both. Because of the ability to 
start heavy loads quickly, series motors are used principally 
for electric cars, cranes and hoists, which do not require a con¬ 
stant speed, f 

147. Shunt Motors. The shunt motor has its armature and 
field windings connected in parallel to the line. The field 


To Line 

+ — 



Fig. 73.—Diagram of Connections for a Shunt Motor. 


winding therefore consists of a large number of turns of fine 
wire which gives a high resistance to the field circuit (Fig. 73). 
This limits the current taken by the field to from 1 to 5 per cent 
of the full-load current of the motor. Because the field is con¬ 
nected directly to the line, the field strength remains constant t 
regardless of the amount of current in the armature. The 
motor will not start as heavy a load as the series motor 
without exceeding a safe current, but the starting effort is 
sufficient for most industrial conditions. Since the field of the 
shunt motor is kept at a constant strength regardless of the 
load, the motor maintains practically constant speed for all 

* See paragraph 175, Chapter 10. 

t See tabulation in paragraph 150. 

t Neglecting the effect of the armature current, which weakens the field 
strength slightly. 




















128 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


loads. With the motors usually employed, the speed drops 
not more than 5 per cent from no load to full load. When in¬ 
terpole motors are used, the drop in speed is from 3 to 4 per cent. 

Speed adjustment may be secured by means of a rheostat in 

%■ 

the field circuit (Fig. 74a). Cutting in resistance and thereby 
weakening the field increases the speed. If the field is weakened 
too much, there is trouble from sparking at the commutator, 
the ordinary shunt motor allowing an increase of only 15 to 30 
per cent above normal speed. By the use of interpoles,* 
a larger speed adjustment may be obtained. For most purposes, 
a motor with a high speed four times the lowest speed (4 to 1 
ratio) is sufficient. Higher ratios may be obtained if necessary. 
Speed adjustment may also be secured by the use of a rheo- 


Field Rheostat Armature Rheostat 



Starting rheostat, line switch and fuses not shown. 


stat in the armature circuit (Fig. 746). Inserting resistance 
in this way reduces the speed, but causes a large loss of power 
in the rheostat. If the speed is reduced 50 per cent, nearly 
half the power taken from the line is lost in the rheostat. If the 
speed is reduced to 50 per cent of normal when carrying full¬ 
load current, it will be about 75 per cent of normal when carry¬ 
ing half-load current. If all the load is thrown off, the speed 
would increase still more and become practically the same as 
if the rheostat was not in circuit. With changing load, there¬ 
fore, the armature rheostat would require continual adjust¬ 
ment in order to maintain a constant speed. In brief, the 
motor with armature rheostat acts somewhat like a series motor, 
and it is therefore not suitable for many applications, such as 
driving lathes and other machine tools which require a constant 
speed. By the use of the field rheostat (Fig. 74a), however, 

* See paragraph 149. 
















PAR. 148 ] 


COMPOUND MOTORS 


129 


the motor speed, having once been adjusted to the proper 
value, will remain constant (with perhaps 5 per cent change) 
regardless of the load on the motor. The loss in the field rheo¬ 
stat is also much less than for the arrangement shown in Fig. 
746. For these reasons, the speed of a shunt motor is usually 
adjusted by means of a field rheostat (Fig. 74a). The use of 
the armature rheostat (Fig. 746) is confined to motors driving 
ventilating fans, blowers and centrifugal pumps, where a speed 
reduction is required, and the load decreases with a decrease 
in speed. When either an armature or a field rheostat is used 
for controlling the speed, it is usually combined with the 
starting rheostat,* although separate rheostats may be used 


To Line 

+ - 



Fig. 75.—Diagram of Connections for a Compound Motor. (Cumu¬ 
lative Compound.) 

for starting and for changing the speed. Adjustment of speed 
can also be made by shifting the armature partially out from 
between the poles, or by changing the position of the field 
poles. These methods have only a limited use. The direc¬ 
tion of rotation of a shunt motor may be reversed by changing 
the connections to either the armature or the field. Since shunt 
motors are constant-speed machines, they are used principally 
for driving machine tools, wood-working machinery, pumps, etc.f 
148. Compound Motors. This motor is a combination of a 
series and shunt motor. It has two field windings, a shunt 
winding connected across the line and a series winding connected 
in series with the armature. The two fields are so connected 
as to give the same polarities, and hence the field strength 
increases as the load increases (Fig. 75). When connected 


* See Fig. 92. 


t See tabulation in paragraph 150, 






















130 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


in this manner, the machine is called cumulative compound 
or sometimes simply compound. The operation of this motor 
depends upon the relative strength of the two fields. If the 
series field is weak, the machine acts more like a shunt motor 
(constant-speed type), and if the series field is strong it acts 
like a series motor (varying-speed type). The compound 
motor will start heavier loads than the shunt motor, but it is 
not quite as good in this respect as the series motor. On the 
other hand, there is no danger of the compound motor reaching 
a dangerously high speed if the load is entirely thrown off. 
The drop in speed as the load increases is greater than for the 
shunt motor (15 to 25 per cent change from no load to full 
load), but it is not as great as the series motor. Speed adjust¬ 
ment, when required, is accom¬ 
plished by means of a rheostat 
in the shunt-field circuit (Fig. 76). 
The direction of rotation may 
best be changed by reversing the 
polarity of the armature terminals. 
If the reversal of rotation is 
made by changing the shunt 
terminals, the series terminals 
must also be reversed so that 
the two field windings will con¬ 
tinue to act together and not 
oppose each other. Compound motors, with relatively weak 
series fields (about 10 to 15 per cent series winding) will 
start fairly heavy loads. They are used for planers and 
large printing presses. Motors with a stronger series field 
(usually about 20 to 50 per cent series winding) give a larger 
drop in speed. They are used for punch presses, shears, bend¬ 
ing rolls, etc., which have heavy momentary loads. To equalize 
the load on the motor, heavy flywheels are used. When the 
load is applied, the motor slows down and the flywheel helps 
carry the load. After the heavy load is removed, the motor 
speeds up the machine ready for the next stroke. If the series 
winding of a compound-wound machine is reversed, so that it 
opposes the shunt winding, the field strength is decreased 
as the load is increased, and we have a differential motor. 
The effect of this is to maintain a constant speed, w T ith changing 
load, the difference in speed between no load and full load being 



Fig. 76.—Connections of 
Compound Motor for Speed 
Adjustment. 

Starting rheostat, line switch and 
fuses not shown. 









PAR. 149 ] 


INTERPOLE MOTORS 


131 


less than for an ordinary shunt motor. It is possible, however, 
to design shunt motors with interpoles* so that the speed will 
be practically constant. There is therefore very little need for 
differential motors, and they have the disadvantage that the 
speed is likely to be unstable at heavy loads. They are there¬ 
fore not used in practice. The series-shunt motor is a com¬ 
pound machine with a very heavy series and a light shunt wind¬ 
ing. The speed characteristic is nearly like a series motor, the 
shunt winding, however, limiting the no-load speed to from 60 to 
100 per cent higher value than the full-load speed. The motor 
is used where a varying speed characteristic is desired and where 
there would be a possibility of overspeeding if an ordinary 
series motor was used. Series-shunt motors are used for 
auxiliary service in steel mills and for crane motors. 

149. Interpole Motors. D.C. motors are frequently built 
with small poles (interpoles or commutating poles) placed be- 


To Line 



Fig. 77. —Diagram of Connections for an Interpole Motor. (Com¬ 
pound Wound.) 

The same connections would apply to a shunt interpole motor with compen¬ 
sating winding. See par. 149. 


tween the main poles. Interpoles are used with series, shunt 
and compound machines. Fig. 77 shows the connections for 
a compound interpole motor. The connections for a series 
interpole motor would be the same with the shunt field omitted. 
For a shunt interpole machine, the series winding on the main 
field poles would be omitted except where a compensating 
winding is used. This is described later in this paragraph. 
Fig. 77 shows a four-pole motor, with only two interpoles. 
This arrangement is used for motors up to about 50 hp. capacity. 

* See paragraph 149. 























132 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


For larger sizes, there would be as many interpoles as there were 
main poles. These interpoles reduce the tendency to sparking 
on overloads, and in the case of shunt-wound motors they keep 
the speed more nearly constant at all loads. Interpoles are 
used particularly for shunt motors which must have a wide 
speed adjustment, and for motors which must be frequently 
reversed, so that satisfactory commutation may be secured 
without shifting the brushes when operating in either direction. 
Interpoles are also of service for motors which are subjected 
to heavy overloads, and where otherwise there might be trouble 
from sparking. The starting and running characteristics of 
series, shunt and compound motors, as described in paragraphs 
146, 147 and 148, apply also to these motors when equipped 
with interpoles. The interpole winding generally used on 
shunt-wound machines is of such a strength as to produce a 
very small speed change with varying load and in some cases 
the speed at full load is higher than at no load. This is likely 
to cause unstable operation, with heavy surges of current and 
changes of speed. To correct this, a compensating winding 
is used. This is a light series winding, consisting of a few turns 
placed on the main field poles and connected in series with 
the armature (Fig. 77). This winding is so connected that its 
polarity is the same as the main shunt winding and hence it 
assists this winding and produces stable running conditions. In 
general, for industrial applications, interpole motors are com¬ 
monly used. The direction of rotation of interpole motors 
may be reversed in the manner already described,* but in doing 
so the interpole winding should always be considered as a part of the 
armature circuit and the polarity of armature and interpole winding 
should always le kept the sims. If the connections of the inter¬ 
pole winding to the armature are reversed, the machine will 
spark badly. 

150. Applications of D.C. Motors. The applications of the 
various kinds of d.c. motors are determined by their performance 
when starting and running. The following tabulation gives 
this information, together with information regarding the prin¬ 
cipal kinds of machines for which they are best suited. This 
table may be used to select a motor for driving a particular 
machine, provided the starting and running requirements of the 
machine are known. 

* See paragraphs 140, 147 or 148. 


PAR. 151 ] PERFORMANCE OF D.C. MOTORS 


133 


Performance of D.C. Motors 1 


Type. 

Starting Torque. 

Running Performance. 

Applications. 

Series 

Very large for heavy 
currents. Small for 
currents less than 
half full load. 

Speed varies widely 
with load. Motor 
will reach a danger¬ 
ous speed if load is 
all thrown off. 

Electric railways, 
cranes and small 
hoists, small air com¬ 
pressors, small eleva¬ 
tors, propeller fans. 

Shunt 

Less than compound 
motor for large cur¬ 
rents. More than 
compound for small 
values of current. 

Speed practically con¬ 
stant. About 3 to 
5% drop from no 
load to full load. 

Wood-working ma¬ 
chinery, screw ma¬ 
chines, lathes, shap¬ 
ers, drills, blowers, 
centrifugal pumps, 
line shafts, pres¬ 
sure blowers, cen¬ 
trifugal fans, print¬ 
ing presses, convey¬ 
ers. 

Com¬ 

pound 

(cumu¬ 

lative) 

Less than series for 
large currents. Less 
than shunt, but 
more than series for 
small currents. 

Speed falls off rapidly 
with increase in 
load, the amount 
depending on 
strength of series 
winding. 

Punch presses, large 
shears, drop ham¬ 
mers, planers, large 
printing presses, pas- 
s e n g e r elevators, 
bending rolls. 

Com¬ 

pound 

(differ¬ 

ential) 

Very small unless 
series winding is cut 
out when starting. 

Speed may be held 
more nearly con¬ 
stant than with 
shunt motor. Is 
likely to be unstable 
at heavy overloads. 

Special constant- 
speed applications 
for small power. 


1 See also paragraph 188, 


Alternating Current Motors 

151. Polyphase Induction Motors. A polyphase-induction 
motor (either two-phase or three-phase) consists of a stationary 
part or stator containing windings to which the power supply 
is connected, and a rotating part or rotor which has no electrical 
connection with the supply, but in. which current is produced 
by induction, hence the name. The motors are of two kinds, 
one having a squirrel-cage or short-circuited rotor and the other 
using a rotor winding connected to slip-rings. The latter type 
is sometimes called a phase-wound motor. The induction motor 












134 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


is similar in its speed changes to the shunt motor, being essen¬ 
tially a constant-speed machine, although by special means 
the speed may be varied. The motor produces a lagging 
power factor,* but this is quite high at or near full load. At 
light loads, however, the power factor is very low. 

152. Squirrel-cage Induction Motors. In this type of motor, 
the rotor winding consists of heavy bars short-circuited at the 
ends by rings. The absence of any commutator or slip-rings 
makes it the most rugged and reliable type of motor built. It 
is, however, a constant-speed device, being similar to the shunt 
motor in this respect. The speed change with load is about 
the same as the shunt motor, there being a drop of from 


To Line 
12 3 



Fig. 78.—Diagram of Connections for a Squirrel-cage Induction 

Motor. Three-phase. 

Connections are those used for small motors. For large motors an auto¬ 
starter is used. See Fig. 79. 

3 to 6 per cent in speed from no load to full load. The speed 
cannot be as easily changed as in the shunt motor, and if an 
adjustable-speed motor is required the slip-ring type must be 
used. Squirrel-cage motors are used for driving line shafts 
and machinery where the speed does not need to be varied, f 
The arrangement of connections for a squirrel-cage motor is 
shown in Figs. 78 and 79. In some cases, where the speed 
changes with load are not important, but where the motor 
must start heavy loads, as in crane service, a squirrel-cage 
motor with a high resistance rotor is provided. This reduces 
the current required to start the load, but the speed variations 
with changes in load are greater and the efficiency is lower. 

* See paragraph 168. t See tabulation, paragraph 160. 

























PAR. 153 ] 


INDUCTION MOTORS 


135 


The running performance of this motor is somewhat like the 
compound motor. The principal applications are small cranes, 
punch presses, etc.* The direction of rotation of a three-phase 
motor may be changed by reversing any two of the connections 
to the stator. A two-phase (four-wire) motor may be reversed 
by reversing the connections of either of the two phases. A 

To I.ine 



Fig. 79.—Diagram of Connections for a Squirrel-cage Induction 
Motor. Three-phase with Auto-starter. 

Line switch not shown. 


two-phase (three-wire) motor may be reversed by reversing 
the outside wires. 

153. Slip-ring Induction Motors. This type of motor has a 
rotor winding consisting of insulated coils connected usually 
like the armature of a three-phase generator. The terminals 
are frequently connected to slip-rings so that external resistances 
may be connected into the rotor circuit (Fig. 80). In some 
cases the slip-rings are omitted and the resistances are mounted 
inside the rotor frame. By the use of these resistances, the 
amount of current taken from the line when starting a heavy 
load is very much less than that required for a squirrel-cage 
motor. The resistances are short-circuited after the machine 
has reached full speed. The motor then operates practically 

* See tabulation, paragraph 160. 



































136 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


like a squirrel-cage motor and gives a nearly constant speed at 
all loads. If it is desired to change the speed of the motor, 
the resistances may be left in circuit after the motor has been 
started, and if they are properly designed to carry the current 
continuously, the speed may be adjusted to any desired value, 
lower than the normal speed, by changing the amount of resist¬ 
ance. When this is done, the motor operates like a shunt 
motor with resistance in the armature circuit. (Par. 147.) 
Consequently the sp6ed will change greatly if the load changes 
and will rise to practically normal speed if all the load is thrown 
off. In addition, there is a large loss of power in the resistances, 
and hence the motor is very inefficient under these conditions. 
The slip-ring motor is used where heavy loads must be started, 


To Line 



Fig. 80. —Diagram of Connections for a Slip-ring Induction Motor. 


as, for example, air compressors, and also for very large motors, 
where it is necessary to limit the starting current to a minimum 
value. It is also used for varying-speed work such as cranes 
and elevators, and for adjustable-speed duty as required for 
ventilating fans. 

154. Multi-speed Induction Motors. The speed of an induc¬ 
tion motor may be changed by changing the number of poles 
for which the stator is wound. This is accomplished by spe¬ 
cial arrangement of the stator windings. By this method it 
is possible to obtain two and in some cases four different speeds. 
The speed with each connection is nearly constant, regardless 
of the load. The complication of the connections and the small 
number of speeds available make the applications of this type 
of motor rather limited. 

155. Commutator-type Induction Motor. A special form of 
induction motor, called a brush-shifting motor, has been recently 



































PAR. 156] 


SINGLE-PHASE MOTORS 


137 


introduced by the General Electric Company. The stator is 
of the usual type, but the rotor is practically like a d.c/armature 
with commutator and brushes. Power from the line is supplied 
to both stator and rotor (in the latter case, usually by means 
of transformers). The position of the brushes determines 
the speed, and the motor can be started, speed adjusted, or di¬ 
rection of rotation changed by properly shifting the brushes. 
These motors act somewhat like series motors when the speed 
changes. Other types of commutator induction motors have 
also been used to some extent in this country where adjustable 



speed is required. Since they employ auxiliary machines, 
however, they are only adapted for large-size motors. 

156. Single-phase Induction Motors. The foregoing dis¬ 
cussion applies to polyphase induction motors, either two- or 
three-phase. There is a limited demand for single-phase in¬ 
duction motors in small sizes and this has been met in several 
ways. A motor with a single-phase winding on the stator and 
with a squirrel-cage rotor is not self-starting. If started by hand 
in either direction, it will continue to run in the particular direc¬ 
tion chosen, and if the load is not too great will increase in speed 















138 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


until the normal value is reached. For practical use, therefore, 
some means must be employed to start the motor. For very 
small motors, for driving fans and similar purposes, a shaded 
pole is used. This is shown in Fig. 81. This consists of a 
heavy copper ring surrounding a portion of each pole piece. 
The effect of this is to start the motor in the proper direction, 
after which it will quickly reach normal speed. If the motor 
is to run in the opposite direction, the copper ring must be 
put on the other side of each pole. A split-phase arrangement 



Fig. 82. —Diagram of Connections for a Split-phase, Single-phase 

Induction Motor. 

AA. Main field winding. BB. Starting winding. S. Starting switch, wdiieh 
is closed when motor first starts. A centrifugal clutch mounted on the motor 
shaft opens S after motor has come up to speed. 

is also used. In one type the motor has a main stator winding 
and an additional starting winding. By proper design of 
these two windings, there is produced a rotating field some¬ 
what like that in a polyphase motor, and therefore the machine 
is self-starting (Fig. 82). A squirrel-cage rotor is used. The 
starting torque is not as good as for polyphase motors, so that 
for the larger sizes a friction clutch is used. With this arrange¬ 
ment the rotor alone is started, and after the machine has reached 
nearly full speed the clutch acts and throws on the load. Split- 
phase motors of this type are provided with a switch which is 
operated by centrifugal action and cuts out the starting winding 





















PAR. 157 ] 


SYNCHRONOUS MOTORS 


139 


after the motor has started. Motors of this type are made in 
sizes up to 1 hp. The split-phase motor of another manufac¬ 
turer uses a standard three-phase stator winding with a start¬ 
ing box which connects resistance and reactance in the circuit 
when starting. A friction clutch picks up the load after the 
machine has reached the proper speed. The split-phase motor 
is at a disadvantage as regards starting torque, and the fric¬ 
tion clutch frequently gives trouble. This type of motor is 
therefore used in small sizes (up to 1 hp.), the repulsion motor* 
being used for larger sizes. Single-phase induction motors 
are more expensive than polyphase motors and are only used 



Motor is started with field switch open. An auto-starter is used for starting 
at reduced voltage similarly to a squirrel-cage induction motor. 


when a polyphase supply is not available. They are usually 
operated from lighting circuits and therefore most central sta¬ 
tions limit the size to about 5 hp. 

157. Synchronous Motors. These motors are constructed 
essentially like a.c. generators and require direct current to 
excite the fields. They are always operated from polyphase 
systems. Fig. 83 shows the arrangement for a three-phase 
motor. The motors must either be supplied with a separate 
starting motor or else special devices must be provided to make 

* See paragraph 159. 















































140 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


them self-starting. In either ease it is not possible to use them 
where heavy loads must be started. Synchronous motors 
must run at a constant speed, which is fixed by the number of 
poles and the frequency of the power supply. These motors 
are chiefly used for driving large, slow-speed air compressors, 
for motor generator sets and for large centrifugal pumps and 
blowers. The power factor of a synchronous motor may be 
changed by varying the field strength. By producing a leading 
power factor* in the synchronous motor, it is possible to neu¬ 
tralize the lagging power factor produced by induction motors 
connected to the same system. 

168. Single-phase Series Motors. These a.c. motors have 
a series field winding and an armature with a commutator 
and brushes and are in many respects similar to d.c. series motors 
except for certain modifications in construction to adapt them 
for use with alternating current. Motors of this type are 
chiefly used for railway work. The characteristics are very 
nearly the same as those of d.c. series motors. 

159. Other Types of A.C. Commutator Motors. The poor 
starting performance of the ordinary single-phase induction 
motor f has led to the development of a number of different 
types of single-phase motors with commutators designed to 
improve the starting performance. A type made by several 
manufacturers uses a single-phase stator winding with a rotor 
built somewhat like a d.c. armature with a commutator. By 
means of short-circuited brushes, located at proper points on 
the commutator, a starting effect is produced which is suf¬ 
ficient to start a heavy load. Fig. 84 shows the arrangement. 
After the motor has reached nearly full speed, a centrifugal 
device short-circuits the commutator bars, lifts the brushes 
off the commutator, and the machine then operates like an ordi¬ 
nary induction motor. The motor runs at practically constant 
speed. Motors of this type will start under full load with 
from 1.5 to 1.25 times full-load current taken from the line. 
The power factor, under starting conditions, is rather low. 
A modified type of this motor made by the Wagner Company 
(Type BK) gives a higher power factor both starting and run¬ 
ning. The rotor contains a starting winding with commutator 
and a short-circuited (squirrel cage) winding, used when 
running. In this machine, the commutator bars are not 
* See paragraph 323. f Paragraph 156. 


PAR. 159 ] 


SINGLE-PHASE MOTORS 


141 


short-circuited when running at normal speed, as current is 
taken from the commutator and passed through an auxiliary- 
winding on the field in order to improve the power factor. 
Fig. 85 shows the connections of this motor. It is a constant- 
speed device. A somewhat similar motor is made by the 
General Electric Company and is called Type RI (Fig, 86). 



Fig. 84. —Diagram of Connections for a Single-phase Repulsion 

Motor. (Two-pole.) 

The rotor has only a single winding, connected to a commuta¬ 
tor. There are two sets of brushes, the “ energy ” brushes 
(3 and 4), which are short-circuited, and the “ compensating ” 
brushes (5 and 7), which are connected to the compensating 
winding. With small motors and all two-pole motors, there 
is only one set of energy brushes, as shown in Fig. 86. For 
larger four- and six-pole motors, there are two sets. The 
speed changes with load about like a compound motor with 

























142 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


a small series field. Motors of this type are also made for 
reversible service. For this purpose, a reversing winding is 
provided which produces the same effect as shifting the 
brushes. A varying speed type is also made. The speed is 
changed by shifting the brushes. Adjustable-speed motors giv¬ 
ing a 2 to 1 speed .rangQ,are also available. Type RI motors are 
made in sizes up to 15 hp. for constant-speed service and up 
to 5 hp. for varying-speed and adjustable-speed service, Com- 



Fig. 85. —Connections for a Wagner, Unity Power Factor Motor. 

Type BK. 

The stator has a main field winding (1) and compensating winding (8) on each 
pole. The rotor has a squirrel-cage winding (3) and drum winding with com¬ 
mutator (2). Main brushes (6) and (7) are short-circuited. Auxiliary or excit¬ 
ing brushes (4) and (5) supply current to compensating winding when switch 
(11) is closed. This switch closes by a centrifugal device when motor is nearly 
up to speed. With lead connected to (9), the maximum compensation is secured, 
giving a leading power factor at light loads. If lead is connected to (10), normal 
compensation is produced, giving a slightly lagging power factor at light load. 


mutator type induction motors have already been described in 
paragraph 155. 

160. Applications of A.C. Motors. The applications for 
which the various types of a.c. motors are best adapted are 
indicated in the tabulation following. In many cases they may 
be used for the same purposes as d.c. motors, the kind chosen 
depending upon the power supply available. For industrial 
applications either three-phase or two-phase induction motors 
are usually employed, synchronous motors being used only in 
large sizes and special applications, and the various single- 



































PAR. 161 ] PERFORMANCE OF A.C. MOTORS 


143 


phase motors being used in small sizes where only single-phase 
power is available. 

161. Vertical Motors. While motors arranged for mounting 
with the shaft vertical can be obtained in various sizes, they 
are to be avoided whenever possible. These motors require 
some kind of a thrust bearing to carry the weight of the arma- 
ture, and this bearing /is more likely to get out of order than the 



Fig. 86.—Connections for a Compensated Repulsion Motor. 

Type RI. 

The figure shows a four-pole motor. Direction of rotation is changed by 
shifting the brushes to the other running mark on the brush-holder yoke. 


bearings of horizontal motors. Vertical motors and their 
repair parts are usually not kept in stock by local dealers, so 
that repairs may be delayed by the necessity of ordering from 
the manufacturer. 

162. Comparison of A.C. and D.C. Motors. It will be seen 
from the foregoing brief descriptions of the various types of 
motors that the usual power applications can be met by either 
a.c. or d.c. motors. There are, however, certain advantages 
and objections for each class. For supplying power to large 
areas, an a.c. system has many advantages, because a high 
voltage may be used to transmit the power, while by means of 


































144 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


Performance of A.C. Motors 1 


Type. 

Starting 

Torque. 

Running 

Performance. 

Applications. 

(1) Two or three- 
phase. Squir¬ 
rel cage (low 
slip). 

Relatively low. 
Starting cur¬ 
rent high. 

Speed nearly con¬ 
stant. About 3 to 
6% drop from no 
load to full load. 
Speed not adjust¬ 
able. 

Small blowers, ce¬ 
ment and steel 
mills, screw ma¬ 
chines, lathes, 
drills, pumps, con¬ 
veyors, wood¬ 
working machin¬ 
ery. 

(2) Squirrel cage 
(high slip). 

Higher than for 
(1). Starting 

current less. 

Speed decreases 
rapidly with load, 
somewhat like 
compound, d.c. 
motor. 

Small cranes and 
elevators, punches 
and shears, large 
band saws. 

(3) Slip-ring. 

« 

Higher than (1) 
and (2). Start¬ 
ing current 
small. 

With starting re¬ 
sistance out, acts 
like (1). With re¬ 
sistance in c i r- 
cuit, speed can be 
adjusted to any 
desired value at a 
sacrifice in effici¬ 
ency. 

Elevators, cranes, 
air compressors, 
ventilating fans, 
steel mills, hoists, 
wood-working ma¬ 
chinery. 

(4) Single-phase 
(split-phase). 

Low starting 
torque. Large 
starting cur¬ 
rent. 

Similar to (J.). 

Used only in small 
sizes. Employed 
for constant speed 
applications only, 
such as small 
printing presses, 
sewing mach., etc. 

(5) Single-phase 
(repulsion). 

Fairly high start¬ 
ing torque, but 
not as good as 
polyphase mo¬ 
tors. 

Similar to (1). 

For constant speed 
applications. Used 
principally in sizes 
below 15 hp. where 
only single-phase 
service is avail¬ 
able. 

(6) Synchronous. 

Low starting 
torque, large 
current. 

Speed constant and 
cannot be adjust¬ 
ed. 

Large air compres¬ 
sors, line shafts, 
pumps. 




1 See also paragraph 188. 
















PAR. 162 ] 


COMPARISON OF MOTORS 


145 


transformers a lower voltage may be used for the motors. This 
reduces greatly the cost of the feeder system. A comparison 
of a.c. and d.c. motors is given in the following tabulation: 

Comparison of Motors 


Direct Current. 


1. Voltage limited to about 240 

volts, if $ame source is used 
for lighting. 

2. Maintenance higher, owing to 

commutators. 


3. Wide speed adjustment by simple 

means, with high efficiency. 

4. Motors have better starting per¬ 

formance for cranes and eleva¬ 
tors. 

5. Starting current is lower for 

usual types of constant-speed 
motors. 


Alternating Current. 


1. The voltage can be easily trans¬ 

formed, using voltages suitable 
for lamps and motors. 

2. Absence of commutator makes mo¬ 

tor more rugged. It will stand 
larger momentray overloads, there 
is no danger of fire from sparks 
from commutator and it is more 
reliable. 

3. Speed adjustment is difficult and 

motor is less efficient at reduced 
speeds. 

4. Operation is not satisfactory on high¬ 

speed elevators and large cranes. 
Starting current is greater. 

5. Starting current for ordinary type 

is large. Special arrangements 
necessary to reduce it. 


6. A somewhat larger generator is re¬ 
quired for a given motor load. 


In general it may be said that, for industrial purposes, there 
is no need of using d.c. motors except for adjustable-speed 
service for machine tools; for varying-speed service, such as 
cranes, hoists, etc., and for service requiring frequent starting 
and stopping of heavy loads, as in high-speed elevators. The 
greater reliability and ruggedness of the a.c. induction motor 
makes its use generally preferable for other purposes. The 
squirrel-cage induction motor, having no commutator or slip- 
rings, can be operated, without enclosing the frame, in very 
dusty or dirty places or in rooms where there would be danger 
of explosion from a spark, such as flour mills and grain eleva- 












146 MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 

tors. If d.c. motors are used in such places, they must be care¬ 
fully enclosed, and this increases the heating and reduces their 
safe output. With standard squirrel-cage induction motors 
(either two-phase or three-phase) it is possible to obtain a 
torque * at starting which is from 1.5 to 2.0 times the torque 
required at full load and with slip-ring motors from 2.5 to 3.5 
times this torque may be obtained. This meets the ordinary 
requirements. Where motors are used in a plant which covers 
a wide area, it is generally customary to provide direct current 
only for the tools requiring this system and to use alternating 
current for all constant-speed service, thus saving in the cost 
of the feeder system. The direct current would be furnished 
by a motor-generator set driven from the a.c. supply and 
located conveniently near the d.c. load. 

163. Standard Voltages. For d.c. motors, the standard 
voltages are 115, 230, or 550, and for a.c. motors, 110, 220, 
440 and 550 volts are commonly employed, although in some 
cases, for very large motors, 2200 volts are used. The voltages 
given are the values at the motors. The generator voltages 
are about 10 per cent higher to allow for the loss in the wiring.f 
D.c. motors smaller than 0.5 hp. should preferably be operated 
at 115 volts. For industrial purposes, 110-volt induction 
motors are seldom used, the common voltages being 220 or 
440 volts. A voltage of 550 has also been used to some extent. 
In general, 220 volts is preferable for moderate-sized installa¬ 
tions, particularly where the supervision may be in relatively 
unskilled hands. The danger of workmen receiving fatal 
shocks is greater with alternating than with direct current, 
and at 440 or 550 volts this presents a real hazard; a shock 
from 220 volts is seldom fatal. In establishments of con¬ 
siderable size, particularly with large motors, the great saving 
in feeder size with 440 or 550 volts results in their frequent 
use. More complete protection is possible w T ith alternat¬ 
ing than with direct current, so that with careful planning 
of the control devices and first-class w r iring, these higher- 
voltage systems can be made fairly safe. Sometimes 1100 
or 2200 volts are used for a.c. motor drives, but such high 
voltages are adapted only for large motors and require special 
methods of installation of the wiring and control system to 

* That is the force tending to rotate the armature. 

t See paragraph 317. 


PAR. 164 ] AVAILABLE SPEEDS FOR A.C. MOTORS 147 

make them safe. The voltage of motors for driving machine 
tools has been standardized at 110 or 220 volts. 

164. Frequency. The standard frequencies for a.c. motors 
are 25 and 60 cycles, while 40-cycle motors are used to a limited 
extent. In general, 60 cycles is the best frequency because of 
the greater number of available speeds. A.c. motors have 
definite speeds, fixed by the frequency and depending upon the 
number of poles, whereas d.c. motors have greater flexibility 
in this respect. The available no-load speeds for a.c. motors 
for the usual range are given below; 


Available Speeds For A.C. Motors 


Number of Poles. 

No-load Speed, R.P.M. 

60 Cycles. 

25 Cycles. 

2 

3600 

1500 

4 

1800 

750 

6 

1200 

500 

8 ' 

900 

375 

10 

720 

300 

12 

600 

250 

14 

514 

214 


The highest speed is for the two-pole motor, and the speed 
can be made as low as required by providing a suitable number 
of poles. It will be seen by reference to this table that, for 
25 cycles, motors can be built for only three speeds between 
500 r.p.m. and the maximum; whereas for 60 cycles there are 
seven speeds. All of these speeds cannot be obtained from the 
same motor, which, on the contrary, must be built for a partic¬ 
ular speed, and only by special design can it be run at more than 
one of these speeds.* For high-speed motors, the cost of 25- 
cycle motors for approximately the same speed is from 5 to 
20 per cent more than 60-cycle motors, and the 25-cycle motors 
are heavier. For very large, slow-speed service (such as rolling 
mills, air compressors, etc.), 25-cycle motors are better, because 
they are cheaper and more efficient. 

*See paragraph 154. 










148 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


165. Rating and Overload Capacity. Motors are rated at 
the normal output in horsepower which they will deliver under 
specified conditions. Recently attempts have been made to 
rate motors in kilowatts output, but the horsepower rating is 
still used by manufacturers. The rating in kilowatts (kw.) 
can be determined by multiplying the horsepower rating by 
0.746. The horsepower rating can be determined by multi¬ 
plying the kilowatt rating by 1.34. The rating does not of 
course give the total input to the motor, because of the losses, 
so the kilowatt output must be divided by the efficiency of the 
motor to obtain the power input. The rated output of a motor 
is always marked on the manufacturer’s nameplate, together 
with the voltage, full-load current, speed, and frequency if an 
a.c. motor. The horsepower rating given on a nameplate indi¬ 
cates the load which the motor will carry either continuously or 
for a short time, depending upon the service for which the motor 
is designed. In all cases the load which the motor can carry 
is limited either by the heating of the windings or by sparking 
at the commutator. All motors will carry much greater loads 
than their rating. If a steam or gasoline engine is too heavily 
loaded, it will stop, without, however, doing any damage to the 
machine. If a very heavy load is placed on an electric motor, 
it will also stop, but before this happens the current will become 
so large that the motor will burn out. An electric motor there¬ 
fore cannot adjust itself to take only a safe load, but must 
be protected against overloads by fuses or circuit-breakers. 
Consequently the output of a motor or its rating is given as 
the horsepower which the machine will carry without exceed¬ 
ing a safe temperature or without appreciable sparking at the 
commutator. It is apparent that the load which a motor can 
carry for a short time is larger than the continuous-load capacity 
of the motor. Accordingly, when the horsepower of a motor 
is given, it must be stated whether this load can be carried 
continuously or only for a short time. For continuous duty 
the motor must carry its rated load continuously without 
exceeding certain specified temperatures (see Table 20). Unless 
the name plate definitely states otherwise, motors are always 
assumed to be for continuous duty. For short-time duty 
the motor must carry its rated load for definite periods of time, 
for example one-half hour, with periods of rest between oper¬ 
ating periods, sufficient to allow the motor to cool off. Motors 


PAR. 165 ] RATING AND OVERLOAD CAPACITY 


149 


for cranes operate under these conditions. In Table 20 is 
given the operating conditions for various kinds of standard 
motors. The temperatures are given in degrees rise in tempera¬ 
ture above the surrounding air. For example, if a 40° Cent, rise 
is given and the room temperature is 25° Cent., this would make 
the temperature of the winding 65° Cent. (149° Fahr.). If, how¬ 
ever, the room temperature is 35° Cent., the rise in temperature 
would be nearly the same and the actual temperature of the 
winding would become 75° Cent. Damage to the insulation of 
the machine, however, is caused by the actual temperature which 
the windings reach, and therefore, if the room temperature is 
likely to be high, care must be taken to select a motor of ample 
size and thus keep the operating temperature down to a safe 
value. In general, the temperature of the windings should not 
exceed 90° Cent, as measured by a thermometer placed on the 
windings and properly protected. Thus, if the room temperature 
was 35° Cent. (95° F&hr.) a motor carrying 25 per cent overload 
might reach a temperature of 90° Cent., and it would probably 
be best to select a motor somewhat larger, so that it would not 
run too close to the limiting temperature. The temperature 
ratings also depend upon the kind of motor, whether open or 
enclosed. This is indicated in Table 20. While motors rated 
for continuous duty can carry a sustained overload of 25 per 
cent for a definite length of time (one-half hour or more), they 
will carry much larger overloads for a few minutes. In such 
cases, the heating effect is not important, and the limit, in the 
case offd.c. motors, is fixed by the sparking at the commutator. 
By the use of interpoles, the commutation of modern d.c. 
motors has been greatly improved so that momentary overloads 
of 50 to 75 per cent may be carried. The standard over¬ 
load rating is, however, 50 per cent, except in the case of 
small motors (see Table 20). Three-phase and two-phase 
induction motors do not have commutator troubles, but 
they are also limited in the amount of overload they can 
carry. If the load on an induction motor exceeds a certain 
amount, the motor will “ pull out ” and stop and will take 
a very heavy current. For commercial motors, a load of 
from 2.5 to 3.5 times full load can be carried without the 
motor “ pulling out ” and stopping. Synchronous motors 
will carry a load of from 1.5 to 2.Q times full load without 
stopping. 


150 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


166. Open ajid Enclosed Motors. Since the output of a 
motor is usually limited by the heating, it is apparent that the 
various parts should be as freely ventilated as possible. In 
many cases, however, it is necessary to enclose the windings 
to protect them against mechanical injury, excessive dust or, 
in some cases, to prevent communicating fire to inflammable 
materials. Motors for industrial purposes are therefore classi¬ 
fied according to the amount of such protection that is pro¬ 
vided. While there are a number of different classes, they 
may all be grouped as either open, semi-enclosed or enclosed 
motors. In the open motor the windings and rotating parts 
are freely exposed to a circulation of air, which serves to cool 
the windings. This type gives the best ventilation possible, 
and therefore for a given load will be the cheapest type of motor N 
to use. The open type would therefore always be selected where 
the conditions permit, that is, where dust or moisture are not 
excessive. Where there is considerable dust or dirt, motor 
bearings are sometimes made dust proof by the addition of 
felt rings at each end. The semi-enclosed motor is similar to 
the open type except that the openings at each end of the frame 
are covered by gratings or screens, which allow a fairly free 
ventilation, but protect the motor against damage from pieces 
of wood, metal, or other substances which might be dropped 
into it. This arrangement does not protect the motor against 
fine dust which could pass through the openings. A semi- 
enclosed motor will run slightly hotter than an open motor, 
since the covers shut off some of the air circulation. The 
enclosed motor has solid covers closing all openings to the 
inside of the machine. Such a motor is practically dust and 
moisture proof. These motors are used for very severe operat¬ 
ing conditions, where there is a large amount of dust or where 
considerable moisture is present. When d.c. motors are in¬ 
stalled where inflammable dust exists, the enclosed type is 
necessary. When a motor is enclosed in this way there is no 
free circulation of air over the various parts of the windings 
and the entire cooling of the motor must be accomplished by 
cooling the outside frame of the machine. A given size of 
motor would therefore run much hotter if enclosed than if open. 
Stated another way, to carry a given load the motor must be 
much larger if enclosed. To partly offset this difference, 
enclosed motors are allowed to run somewhat hotter than open 


PAR. 167 ] 


MOTOR PERFORMANCE 


151 


motors, as will be seen from Table 20. An enclosed motor 
is from 25 to 40 per cent heavier than an open motor and is 
therefore correspondingly more expensive. In some cases, 
particularly with large enclosed motors, air is blown into the 
motor by means of a blower to assist in the cooling. 


Motor Performance 

167. Motor Currents. The full-load current taken by shunt 
or compound motors is given in Table 21. This table can also 
be used without great error for series motors. For three- 
phase and two-phase induction motors, Tables 22 and 23 may 
be used. The values of current given in the tables take into 
account the efficiency of the motor and in the case of induction 
motors also include the power factor. The values at other 
loads can be approximately determined by multiplying by 
the given load. Thus a 50-hp., 230-volt, d.c. motor oper¬ 
ating at three-quarters load would require a current of approx¬ 
imately 0.75X178 = 134 amperes. This method is not accu¬ 
rate at light loads since the efficiency under such conditions is 
much lower. While the tables give the full-load current for 
squirrel-cage induction motors, the same values may be used 
for slip-ring motors without much error. To determine the 
current for a motor not given in the tables, find the amperes 
per horsepower for the next smallest motor and then multiply 
by the horsepower of the given motor. 

Example. Find current for a 65-hp., 550-volt, d.c. motor. From 
Table 20 the full-load current of a GO-hp. motor is 89.5 amperes. 

89.5 

Amperes per hp. = -^- =1.49. 

Full-load current for a G5-hp. motor is 1.49 XG5 =97 amperes. 

The comparative starting current required by different types 
of motors is indicated in the following tabulation. There is, 
however, considerable variation in this respect for different 
designs. 



152 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


Comparison of Starting Currents for Motors 


Type of Motor. 

Current Compared with Fulu- 
load Running Current. 

For Full-load 
Torque. 

For 50% Over¬ 
load Torque. 

Series. 

1.0 

1.3-1.4 

Shunt. 

1.0 

1.5 

Compound 1 . 

1.0 

1.45 

Squirrel-cage induction 2 (low slip) . . . 

2.5-3.0 

5.7 

Slip-ring induction. 

1.0 

1.5 


1 For a motor with 27 per cent series field. 

2 Current taken from line, using auto-starter. 


The starting current for compound motors depends upon the 
strength of the series field. The value given in the table is for 
a rather weak field, such as would be used for planers, etc. It 
is possible to build squirrel-cage motors which will take only 
slightly more current than the slip-ring motor, but such a 
machine would have a poorer efficiency and wide speed varia¬ 
tions with changes in load. The starting current of a syn¬ 
chronous motor depends upon the service for which it is designed. 
A starting torque of about 25 per cent of full-load torque can 
be obtained with 1.75 times full-load current, requiring about 
one-half normal voltage. This torque is sufficient for the usual 
starting requirements. Single-phase induction motors which 
are started by a set of short-circuited brushes * require about 
1.25 to 1.5 times full-load current for full-load torque. The 
Wagner Type BK motor requires about 2.0 times full-load 
current for full-load torque. The General Electric RI motor 
requires about 2.25 times full-load current. 

168. Power Factor of A.C. Motors, f With a.c. motors the 
power factor must be taken into account. That is, the actual 
power taken by the motor is not the same as the power cal¬ 
culated from the volts and amperes, but is usually less. Thus 
a three-phase, 50-hp., 440-volt induction motor requires 61 
amperes at full load. The apparent power taken from the line 
*See paragraph 159. f See paragraph 323. 














PAR. 169 ] 


MOTOR PERFORMANCE 


153 


is therefore 440X61 X1.73=46,400 volt-amperes. But since 
the power factor of this motor is 0.89, the actual power taken 
from the line would be 0.89X46,400=41,300 watts or 41.3 
kw. 1 he power factors of induction motors when running at 
three-quarters or full load are given in Table 24. At no load, 
the power factor is very low for all sizes of motors, being 
about 0.30. For the usual operating loads, however, the power 
factor is not less than 0.80 to 0.85. These values apply to squir¬ 
rel-cage or slip-ring induction motors, whether three-phase or 
two-phase. When starting full load, the squirrel-cage motor 
has a power factor of about 0.50. The slip-ring motor has a 
much higher power factor at starting. It is practically the same 
as the running power factor for the same current input. The 
power factor of a synchronous motor, when running, can be 
changed by adjusting the field rheostat. When adjusted 
to a value of 1.0 the current taken by the motor is the smallest 
possible for the particular load carried. Weakening the field 
by cutting in more of the field rheostat will increase the cur¬ 
rent and give a lagging power factor, although the real power 
taken by the machine will not change unless the load changes. 
Strengthening the field, on the other hand, will also cause the 
current taken by the motor to increase and will at the same time 
give a leading power factor. Because of this action, the 
synchronous motor is sometimes adjusted to produce a leading 
power factor to correct the lagging power factor produced by 
induction motors connected to the same system. The power 
factor of a synchronous motor at starting is very low, being 
about 0.20. 

169. Effect of Change in Voltage. With d.c. series motors, 
the speed for a given current depends upon the voltage. Hence 
a 10 per cent increase in voltage would result in a speed increase 
of 10 per cent if the current remained the same. If the load 
increased faster than the voltage the speed increase would 
be less than 10 per cent. With shunt motors a moderate change 
in the voltage applied to the motor has very little effect upon 
the speed. The compound motor usually acts in somewhat 
the same way as the shunt motor. The speed of an induc¬ 
tion motor varies only slightly with moderate changes of line 
voltage, but the maximum load which the motor can carry is 
affected greatly by the voltage. As the maximum torque 
varies -with the square of the voltage, small changes in voltage 


154 


MOTORS FOR INDUSTRIAL PURPOSES [CHAP. 9 


produce large changes in torque. Thus a squirrel-cage induc¬ 
tion motor giving, at normal voltage, a maximum torque 2.5 
times full-load torque would if the voltage dropped 20 per cent 
have a torque only 1.6 times* full-load value, and hence might 
stall on an overload. The speed of synchronous motors is not 
affected by changes in voltage, but the torque is affected similarly 
to induction motors. In general a voltage variation of 10 
per cent above or below normal will not interfere with satis¬ 
factory operation. 

170. Effect of Change in Frequency. The speed of induc¬ 
tion motors depends upon the frequency. If the frequency 
is increased 10 per cent, the speed will increase 10 per cent, and 
the opposite effect occurs for a decrease in frequency. This 
assumes that the load on the motor does not change. Induction 
motors will usually operate satisfactorily with a frequency 
variation of not more than 10 per cent above or below normal. 
Synchronous motors maintain absolutely constant speed as 
long as the frequencjr is constant, and hence the speed depends 
upon the frequency, regardless of the load. 


*0.80X0.80X2.5=1.6. 


CHAPTER 10 


MOTOR-STARTING DEVICES AND CONTROLLERS 

Starting Methods 

171. D.C. Motors. Small d.c. motors (up to f hp. for shunt 
and 5 to 8 hp. for series) may be thrown directly upon the line 
without injury. For larger motors, a resistance must be placed 
in the armature circuit to limit the starting current to a safe 
value because of the low resistance of the motor armature. 
For example, a 25-hp., 230-volt, shunt-wound motor would 
require a full-load current of about 91 amperes. The resistance 
of the armature would be about 0.10 ohm. Hence, if the arma¬ 
ture was connected directly to the line without a starting 
resistance, the current would be 230^0.10=2300 amperes or 
over 25 times full-load current. The starting resistance is 
usually arranged to allow about 1.5 times full-load current to 
flow when the motor is first thrown on the line. For severe 
starting service, such as cranes and elevators,, a larger starting 
current may be allowed. In all cases the field strength at 
starting should be as large as possible to give the maximum 
torque for the current used. With shunt and compound 
motors, therefore, the shunt field winding is always connected 
directly to the line without any resistance in circuit. The 
position of the starting resistance would be as shown in Figs. 
73 and 75. As soon as the motor starts, the current taken 
decreases and the starting resistance must be reduced. D.c. 
motor starters are therefore arranged to cut the resistance out 
in several steps as the motor speeds up, until finally all the re¬ 
sistance is out and the motor, armature is directly across the 
line. 

172. A.C. Motors. Small squirrel-cage induction motors 

(up to 5 or 8 hp.) may be started by throwing them directly 
on the line. The current which the motor takes when this is 
done is not as great as for d.c. motors, being from five to eight 
times full-load current. Motors as large as 50 hp. might be 

155 


156 


MOTOR-STARTING DEVICES 


[CHAP. 10 


started in this way without damage to the motor. The heavy 
current required would, however, cause a very large voltage 
drop in the feeders and frequently serious disturbance of the 
generator voltage. For starting small §quirrel-cage induction 
motors, resistance starters (Fig. 99) are sometimes used, but 
the auto-starter or compensator (Figs. 97 and 98) is usually 
employed. A star-delta method of starting is also employed 
to some extent (Fig. 101). Slip-ring motors are started by 
inserting a resistance in the rotor circuit (Figs. 80 and 100). 

173. Auxiliary Apparatus. All motors must be provided w r ith 
a switch to disconnect the motor from the line and with fuses 
or a circuit-breaker, to protect the motor in case of heavy over¬ 
loads. These devices are described in Part III. 

174. Rating of Starters. Starters are rated according to the 
sizes of motors for which they are designed. A different 
starter is required for different voltages and sizes of motors. 
With d.c. motors, if the starter is too large, the current taken by 
the motor will be excessive; if too small, the motor may not 
start and the starter may be burned out. If a small auto¬ 
starter is used with a large a.c. motor, the starter will be over¬ 
loaded and probably damaged. Ordinary starters are intended 
for occasional use only and do not have sufficient capacity to 
carry the starting current continuously or to start the motor 

. at very frequent intervals. If the re¬ 

sistance is to be left in the circuit for 
adjusting the speed as shown in Fig. 72, 
the rheostat is called a speed regulator. 

D.C. Starting Rheostats 

175. Starters for Series Motors. 

For motors which are to be started 
only at fairly long intervals, a starter 
with norvoltage release, similar to that 
described in paragraph 176, is used. 
Where motors are to be frequently 
started, stopped and reversed, as in 
crane service, either face-plate starters 
(Fig. 87) or drum controllers (Fig. 
93) are used. The motor is reversed 
and brought up to speed by movements of a single handle. 



Fig. 87.—Starter for 
Series Motor. 

Small crane or hoist con¬ 
troller. 





























PAR. 176 ] 


D.G. STARTING RHEOSTATS 


157 


176. Starters for Shunt and Compound Motors. The sim¬ 
plest form of starter for these motors is the hand-operated 
face-plate starter (Fig. 88). This includes a suitable starting 
lesistance, so connected that it may be short-circuited by a con¬ 
tact arm as the motor is brought up to speed. This type of 
starter is used for motors up to about 50 hp. For large motors, 
the multi-switch type (paragraph 177) is used. In Fig. 88 
a face-plate starter is shown connected to a compound motor. 
The same type of starter can be used for shunt or series motors. 
These starters are provided with a low-voltage release. This 
consists of an electro-magnet which holds the rheostat arm in 


To Line 

+ 



Fig. 88. —Connections of Starter for Shunt and Compound Motors. 

With low-voltage release. 


the running position as long as there is voltage on the motor 
circuit. In case of a failure of the line voltage, the magnet 
releases this arm and it is returned to the off position by a spiral 
spring. Hence the motor will not be damaged if the voltage 
is thrown on again. The magnet does not release until the motor 
has slowed down, because, for a short time, the motor acts as a 
generator and keeps the magnet energized. The low-voltage 
release magnet is either connected directly across the line (as 
in Fig. 88) or is placed in series with the shunt field winding. 
When connected across the line, the strength of the magnet is 
independent of the current in the motor field. If connected 
in series with the shunt field, the magnet might become so weak 
(due to weakening the shunt field) that it could not hold the arm 




























158 


MOTOR-STARTING DEVICES 


[chap. 10 


in the running position. If the magnet is in the field circuit, 
however, it will release and stop the motor if the field circuit 
is opened, whereas if connected across the line it would not 
release and the motor might run away. Usually in such cases 
there is sufficient load on the motor to prevent it from running 
away and the heavy current taken by the motor would be suf¬ 
ficient to open the circuit breaker or blow the fuses. Starters 
of this type are frequently equipped with an Overload release. 

This consists of a coil 
in series with the ar¬ 
mature which oper¬ 
ates similarly to a 
circuit breaker. Fig. 
89 shows a starter 
with overload and rio- 
voltage release as 
made by one manu¬ 
facturer. The over¬ 
load release is only 
used on’ compara¬ 
tively small motors 
(up to about 15 hp.). 
For larger motors, 
separate circuit 
breakers are used. 
For many industrial 
plants, particularly 
where there is con¬ 
siderable dust or 
where it is desirable 
to protect the opera¬ 
tors against possibility of an electric shock, the starters are 
enclosed in metal boxes. The face-plate type of starter is 
used where the motor is started only occasionally. In machine 
tool service, etc., the drum type controller* is more satis¬ 
factory because it will withstand harder usage. 

177. Multiple-switch Starters. The face-plate starters de¬ 
scribed in the previous paragraph are not suitable for large cur¬ 
rents because of difficulties with the contacts. The multiple- 
switch starters, which are used for large motors, consist of a 

* Sec paragraph ISO. 



Fig. 89.—Connections of Starter for D. C. 
Motors. With overload and low-volt- 
age release. 

Connections shown are for a compound motor. 
For a shunt motor, A is connected directly to 
armature. For a series motor, connection to F is 
omitted. When arm is on the starting-point (1), 
the motor receives full field. As the armature 
resistance is cut out by moving arm to the 
right, this resistance is cut into field circuit. The 
value of this resistance is so low, however , com¬ 
pared with that of the field, that it has no effect 
upon the speed of the motor. 




































PAR. 178 ] 


D.C. STARTING RHEOSTATS 


159 


number of switches each short-circuiting a section of the 
starting resistance. The general arrangement of a multiple- 
switch starter is shown in Fig. 90 and the connections in Fig. 
91. The switches are mechanically interlocked so that they 
must be closed in regular order, corresponding to the move¬ 
ment of the arm of the face-plate starter. One of the switches 
acts as a circuit breaker and is provided with a low-voltage 
release which acts in the manner already described. This 



Fig. 90. —Multiple Switch Starter. 

For large d.c. motors. 

switch is also provided with an overload release to protect the 
motor. 

178. Speed Regulators. When the speed of a motor is to be 
adjusted by an armature resistance (Figs. 72 or 746), a rheostat 
somewhat similar to that shown in Fig. 88 is used, except that 
provision must be made to hold the arm on any one of the con¬ 
tact buttons. To comply with the Code rules, this arm must 
have a low-voltage release, to stop the motor if the voltage 
fails. An overload release is also used in some cases. If a 








































































































































160 


MOTOR-STARTING DEVICES 


[chap. 10 


speed-regulator of this type is used, no additional starting 
rheostat is required. When the speed of a shunt or compound 
motor is adjusted by field control (Fig. 74a), a separate rheo¬ 
stat may be used. It must be of such a type that the field 
circuit is always kept closed, otherwise the motor would tend 
to run away. When a rheostat of this type is used, a starting 
rheostat must also be provided. This would be either of the 


Resistance 



Fig. 91. —Connections for Multiple Switch Starter. 


To start the motor, the line switch is closed, thus completing the circuit to 
the motor through the entire resistance. Switches R2, R3, etc., are closed in 
regular order, thus cutting out the starting resistance. These switches are 
mechanically interlocked, so they can only be closed in the proper order The 
pendant switch must be held closed until R 6 is closed, otherwise the low-voltage 
release relay will trip the line switch and open the circuit. 


two types previously described. To ensure that the motor 
shall always be started with strong field* the starter is provided 
with a relay which short-circuits the field rheostat while the 
motor is being started. Generally the starting rheostat and 
the field rheostat are combined, forming a compound starter. 
Speed regulators are used principally on ventilating fans. 

179. Compound starters are arranged to cut in the field re¬ 
sistance after the motor is up to speed (Fig. 92). Movement of 
the arm towards the running position first cuts out the starting 

* For the reason given in paragraph 176. This is required by the Code. 



















































PAR. 180] 


D.C. STARTING RHEOSTATS 


161 



Fig. 92. —Connections for a Compound Starter. For starting 
and adjusting speed of shunt and compound motors. 

The upper row of buttons is connected to the field resistance, the middle 
row to the armature. The curved segment below the buttons is used to short- 
circuit he field resistance when starting. There are two movable arms (a) 
and (6) pivoted to the same hub. These move together when the motor is 
being started. Arm (a) makes contact with both rows of buttons and (b) 
with the segment. When the arms have reached the running position (to the 
extreme right) arm (a) can be moved back, leaving ( b ) as shown. This inserts 
resistance in the field circuit. A low-voltage release coil (c) causes both (a) 
and (6) to return to the off position if the supply fails. 


resistance with full field on the 
motor. After the starting resistance 
is all out, further movement of the 
arm inserts resistance in the shunt 
field. Figs. 93 and 94 show a con¬ 
troller of this type used for operation 
of machine tools. 

180. Drum Controllers. For ad¬ 
justable speed, shunt or compound 
motors operating machine tools, the 
drum controller is best (Figs. 93 and 
94). This contains contacts for 
starting the motor and also contacts 
for inserting field resistance, as in 
compound starters. These control¬ 
lers are also arranged to reverse 
the motor by a change in the di¬ 
rection of motion of the handle. 
They are used in capacities up to 
about 50 hp. For larger motors 
the controller operates electro-mag- 



Fig. 93. — Machine-tool 
Controller. (Cover 
removed.) 


With contacts for starting, 
reversing and adjusting speed 
by field control. 10-horse¬ 
power capacity. 



































































































162 


MOTOR-STARTING DEVICES 


[CHAP. 10 


netic switches, mounted separately, which make the hecessary 
connections for starting. 

181. Automatic Starters. The starters previously described 

■* c, 

are all hand operated, and in the hands of a careless workman 
may be so operated as to damage the motor by starting too 
quickly or to burn out the starter by running too long in the 
starting position. There are. also cases where the motor must 



TOP 


Reverse 


Off 


Forward 


Shunt Fid. 


G 54 3 2 1 

III! 


2 46 8 10 12 14 16 
0 1 3 5 7 9 11 13 15 

I I • I ' I M ' I ' I | I | » | 

) A 1 ! i: I 1 ' ' ! ! ! I !I ' 


Aux. Fid. 


L- 


(lontrollei 


Vux. Fid. 



L+ 


■*5W 


Tri 

'T?l 


3 


yX? | R 2 


'Fid. Res. 

Shunt Fid. 


SCHEME OF CONNECTIONS 
WHEN USED WITH 
INTERPOLE MOTOR 






S Shunt Fid. 



r°nH P. 


L __^T2iUULDJ 

1 Shunt Fid. 


R2 


Fid. Res 

SCHEME OF CONNECTIONS 
WHEN USED WITH SHUNT MOTOR 


w 





Shunt Fid. Controll^ 
Ser. Fid. 

Ao Ai 


Main Res. 

PLAN VIEW OF FACE PLATE AS 
SEEN FROM HANDLE END OF CONTROLLER 


Ser. Fid, 



Id. Res 


Shunt Fid. 


SCHEME OF CONNECTIONS 


WHEN USED WITH COMPOUND MOTOR 


Fig. 94. —Connections of Machine-tool Controller, Shown in 

Fig. 93. 


This controller has 16 running-points forward and 6 reverse. In addition 
there are two starting-points. OVestinghouse Electric & Mfg. Co.) 


be started and stopped in response to changes in pressure, 
water level, etc. The simplest type of controller to meet these 
conditions is the dash-pot type (Fig. 95). The rheostat arm 
is moved by a solenoid magnet which is retarded by a dash 
pot, so the resistance will not be cut out too rapidly. This type 
of starter is used for small motors. It may be controlled by a 
switch or push buttons located at any convenient point or by 
means of a contact-making pressure gauge or a switch operated 






























































































































PAR. 181 ] 


D.C. STARTING RHEOSTATS 


163 


by a float. For larger motors, it is usually necessary to employ 
a starter which will automatically limit the starting current to 
a safe value. While there are many arrangements to secure 
this result, the principle can be understood by reference to Fig. 
96. The operation is controlled by push buttons. Closing 
the “ start ” button completes a circuit from the — line through 
1, 10, 01, 0, B, back to the + line. This closes the line 
switch (No. 1), completing a circuit from the + line through 



Fig. 95.—Automatic Starter. Dash-pot type. 

When control switch is closed, line switch solenoid is energized through 
Li, L, C, H, D to L\. This closes line switch and motor is started with all 
resistance in. When the line switch closes, the auxiliary contacts close the main 
solenoid circuit from Li to A and thence to Li. This causes the rheostat arm 
to rise slowly as controlled by the dash-pot and thereby cut out the resistance. 
As soon as the arm starts, the circuit through the line-switch solenoid becomes 
Li, L, C, H, D, B, Li, thus inserting additional resistance so as to reduce the 
current consumed by the solenoid. When all starting resistance is cut out, a 
contact brush on the movable arm makes a direct connection between Li and A i. 
At the same time, additional resistance A-B is inserted in the solenoid circuit. 
Motor is stopped by opening control switch. In place of a simple control switch, 
either a push button or a pressure gauge may be used, with connections as shown 
in the auxiliary diagrams. (Cutler-Hammer Mfg. Co.) 


B, C, R 3 , R <2 , Ri, A h S 2 , to the — line. The closing of the line 
switch (No. 1) closes contacts 10 and 1, thus keeping a circuit 
through the operating coil of No. 1 even when the “ start ” 
button is released. The starting current passes through the 
operating coil of switch No. 2, which is so designed that it is 
locked open until this current drops to a predetermined value. 
Switch No. 2 then closes, thus short-circuiting resistance Ri~R 2 . 
Switch No. 3 has an operating coil connected in shunt with the 





















































































164 


MOTOR-STARTING DEVICES 


[CHAP. 10 


armature terminals. The voltage across the armature is low 
at start, and hence this switch remains open. It is so adjusted 
that it will not close until after switch No. 2 has closed and the 
motor has speeded up. If the motor is overloaded while run¬ 
ning, the relay (R) opens the control circuit of the line switch 
(No. 1) thus disconnecting the motor from the line. To stop 
the motor, the “ stop ” button is pushed. This opens the line 
switch-control circuit, which causes the line switch to open. 
Switch No. 1 always opens before No. 3 because the latter is 
across the armature and receives voltage until the motor has 
slowed down. Only switch No. 1 is therefore provided with 
a blow-out coil for breaking the arc when the switch opens. The 
connections shown in Fig. 96 are for a compound motor with 
compensating winding. For shunt motors, A 2 and F 2 are con¬ 
nected to the — line. 

182. Dynamic Braking. If a motor driving an elevator or 
other load having considerable inertia is disconnected from the 
line, the load will drive the motor. Under these conditions a 
shunt motor will become a generator without any change in the 
connections. A resistance connected across the armature will 
absorb power from the load and slow down the machine. The 
same effect can be produced with series motors if the field 
winding is connected temporarily to the line (through a suitable 
resistance). This action is used either to make a quick stop 
or to retard a descending load. Dynamic braking for making 
a quick stop is employed for elevators, printing presses and 
machine tools. The controller is arranged to connect a resist¬ 
ance to the armature circuit after it is disconnected from the 
line. The field circuit is kept connected to the line to give a 
high braking effect. As the motor slows down, the voltage 
generated by the armature decreases, and hence the current 
through the resistance would decrease. To obtain the greatest 
braking effect, therefore, provision must be made to reduce the 
resistance as the machine slows down. The final stop is made 
by a friction brake which is controlled by an electro-magnet. 
Dynamic braking for retarding a descending load is employed 
on cranes and ore-handling machinery. The motors are gener¬ 
ally series wound, so the field winding is placed across the line 
in series with a suitable resistance. The resistance across the 
armature is then adjusted until the required speed is secured. 
Sometimes the armature is connected to the line in series with 


PAR. 182 ] 


D.C. STARTING RHEOSTATS 


165 





































































































































166 


MOTOR-STARTING DEVICES 


[chap. 10 


a resistance. In this case ; the machine will return power to 
the supply. 

A.C. Starting Devices 

183. Auto-starters or Compensators are used for squirrel- 

cage induction motors. They consist of special single-coil 
transformers (auto-transformers) provided with a two-throw 
switch by which the motor can first be put on a reduced voltage 
and then thrown on to full voltage. At the same time, the auto- 



Fig. 97. —Connections for Three-phase Motor with Auto-starter. 

To start the motor, contacts on *' start ” side are closed. After the motor 
has reached the proper speed, the starter is thrown to “ run ” position, which 
opens “ start ” contacts and closes “ run ” contacts. This cuts out the auto¬ 
transformers and feeds the motor through the running fuses. See Fig. 79 for 
simplified diagram. Starting voltage can be adjusted by changing connec¬ 
tions at taps A 2 , Br, A 3 , Bz, etc. The starting fuses are usually located at the 
point where the branch is connected to the mains. Motor can be reversed by 
interchanging any two leads at motor. Low-voltage rplease and overload 
relays not shown. 


transformers are disconnected from the line. The switch for 
producing these changes has its contacts immersed in oil to 
reduce the arcing. The principle of operation is illustrated 
in Fig. 79 and the actual connections for one make of starter 
are shown in Figs. 97 and 98. Starters for small motors (up 
to about 20 hp.) have taps giving 50, 65, or 80 per cent of normal 
voltage at starting. In order to keep the starting current down 
to a minimum, it is important to select the lowest voltage tap 
which will start the load. When connected to the 65 per cent 
tap, most squirrel-cage motors will give nearly full-load torque. 
Auto-starters are generally provided with a low-voltage release 



























































PAR. 183 ] 


A.C. STARTING DEVICES 


167 


which consists of a magnet connected across the line terminals- 
This magnet, when the voltage fails, trips a catch which holds 
the starting lever in the running position. The lever is then 
returned to the off position by means of a spring. Some¬ 
times an overload release is also provided. This consists of 
two magnets connected in the motor circuit and arranged to 
open the low-voltage release circuit when a heavy current flows. 
Usually, however, the overload release is not used, fuses being 
employed to protect the motor. By reference to Figs. 97 



Fig. 98.—Connections for a Two-phase Motor with Auto-starter. 

To start the motor, contacts on “ start ” side are closed. After the motor 
has reached the proper speed, starter is thrown to the “ run ” position, which 
opens “ start ” contacts and closes “ run ” contacts. This cuts out auto- 
transformers and feeds motor through running fuses. Starting voltage can be 
adjusted by changing connections at taps A 2 , Bi\ A 3 , Bz, etc. The starting 
fuses are usually located at the point where branch is connected to the mains. 
Motor can be reversed by interchanging either Ai and A 2 or B\ and Bi. For 
a two-phase, three-wire line, use connections for three-phase starter, connecting 
L\ and La to the common wire. Motor can be connected with four leads as in 
diagram above or with three leads. In the latter case, motor leads A 1 and Bz 
are connected together to starter leads Ai and Bi. To reverse motor, inter¬ 
change leads A 2 and B\ at the motor. Low-voltage release and overload relays 
not shown. 


and 98 it will be seen that the running fuses are not in circuit 
when the lever is in the starting position. If the fuses had to 
carry the starting current (which is usually for four or five 
times full-load current) they would not protect the motor when 
running. The fuses are therefore chosen to allow only a safe 
overload. This leaves the motor unprotected when starting, 
except for the fuses which protect the branch wiring.* Auto¬ 
starters are not used with slip-ring motors. 

*“See paragraph 331. 



































































168 


MOTOR-STARTING DEVICES 


[CHAP. 10 


184. Resistance Starters. Two-phase and three-phase squir¬ 
rel-cage motors may be started by means of resistance in series 
with the motor. The arrangement is shown in Fig. 99. When 
a resistance starter is used, the motor takes more current from 
the line than when an auto-starter is used. This can best be 
explained by an example. A squirrel-cage induction motor 


Line 



Fig. 99.—Connections of a Resistance Starter for Squirrel Cage 

Induction Motors. 

Connections as shown are for a two-phase, four-wire system. For a two- 
phase, three-wire system, omit L\ and use L% as common lead. For three-phase, 
omit Li. 

requires about 65 per cent of normal voltage to start full load 
and takes about 4.5 times full-load current in the motor. 
Thus a 25 hp., 220-volt, three-phase motor takes 62.6 amperes 
at full load. (Table 22.) Hence at starting, the motor would 
take 62.6X4.5=281 amperes. The voltage applied to the 
motor should be 220X0.65 = 143 volts. If a resistance is used, 
it must be large enough to carry 281 amperes and must consume 
the difference between 143 volts and 220 volts. The current 






































PAR. 184 ] 


A.C. STARTING DEVICES 


169 


taken from the line would be 281 amperes. If an auto-starter 
is used, the required voltage (143 volts) would be obtained by 
connecting the motor to proper taps on the transformers. 
When this is done, the current in (he motor is 281 amperes as 
before, but the current taken from, the line is only ^|fX281=183 
amperes. This results from the transformer action and hence 
gives a decided advantage in favor of the auto-starter. If 
the load is small 
enough to allow 
a lower voltage to 
be used in starting, 
a still greater ad¬ 
vantage results. 

The resistance 
starter also wastes 
more power than 
the auto-starter. 

Because of these 
facts, the resist¬ 
ance starter is 
seldom used for 
squirrel-cage mo¬ 
tors, but is em¬ 
ployed more fre¬ 
quently for the 
single-phase com¬ 
mutator type. The 
principal advan¬ 
tages of the re¬ 
sistance starters 
are their low cost and their simplicity. These starters are 
usually provided with a low-voltage release attachment which 
is connected across the line. Resistance starters are com¬ 
monly used for slip-ring induction motors. With these 
motors, the stator is connected directly to the line, through 
a suitable fuse and switch. The resistances are connected in 
the rotor circuit. The arrangement of connections is shown 
in Fig. 80 and a view of a starter of this type is shown in Fig. 
100. At starting the resistance is all in circuit, and as the motor 
speeds up the resistance is cut out in steps until finally the 
slip-rings are short-circuited, and the motor operates practi- 


Contact 

Buttons 


Resistances- 



Fig. 100. —Resistance Starter for Slip-ring 
Induction Motor. 


















































































170 


MOTOR-STARTING DEVICES 


[CHAP. 10 


cally like a squirrel-cage motor. If the starting resistance is 
made large enough, it can be left in circuit and the speed of the 
motor adjusted by changing the resistance. 

185. Star-delta Starting. The necessary reduced voltage 
for starting a squirrel-cage induction motor can be produced 
in effect by rearranging the motor windings. Thus, in a three- 
phase motor, the three groups of coils may be arranged in star 
(or Y) as shown in Fig. 101a. Each winding then receives 



Starter 

Fig. 101. —Diagram of Star-delta Method of Starting Squirrel-cage 

Induction Motors. 

When switch is in “ start ” position, center contacts connect with correspond¬ 
ing contacts at right, marked start. After the motor has speeded up, center 
contacts are thrown to left to “ run ” position, where they are held by the 
low-voltage release coil. 

0.58 of the line voltage, or in this case 220X0.58 = 127 volts. 
After the motor has started, the windings are thrown directly 
across the line by means of a switch. (Fig, 1016.) The 
voltage across each winding is thereby raised to 220 volts. 
If the load is not too great, this method is satisfactory, but the 
starting voltage cannot be adjusted, so that for heavy loads the 
motor would not receive enough voltage to start. This arrange¬ 
ment is limited in its application to small sizes of motors, where 
the starting loads are not heavy. The switch used with this 






















































































PAR. 186 ] 


A.C. STARTING DEVICES 


171 


method of starting is usually provided with a low-voltage re¬ 
lease attachment, similar to that used with auto-starters. 

186. Starting Switches and Fuses. Since squirrel-cage 
induction motors require large starting currents, some pro¬ 
vision must be made to cut out the fuses when the motor is 
starting, otherwise they would be blown. For small motors 
which do not require starters, a two-throw switch is used. 
When the switch is thrown in the starting position the motor 
is connected to the line without fuses. After the motor has 
started, the switch is thrown into the running position, which 
puts the fuses in series with the motor. These fuses are made 
of proper capacity to give protection against overloads. In 
order to prevent the switch being left in the starting position, 
thus leaving the motor unprotected, springs are provided to 
open the switch if the handle is released while in the start¬ 
ing position. Fig. 78 shows this arrangement. While the 
switch does not have fuses in the starting position, the motor 
is protected to some extent by the fuses protecting the branch 
motor circuit, which must be fused for the starting current. 


CHAPTER 11 


SELECTING MOTORS FOR INDUSTRIAL PURPOSES 

187. Methods of Driving Machines. When machines are 
driven electrically, either group or individual drive may be used. 
Group drive is a modified form of mechanical drive. The 



Fig. 102.—Individual Drive. 

Showing entirely enclosed motors belted to small circular saws. Controllers 
are automatic and are entirely enclosed. Motor is started by closing the switch. 

main shafting and belts are eliminated and machines of a 
similar kind, arranged in groups, are belted to short lengths 
of comparatively light shafting. Each group is then driven 
by a motor. By this means the friction losses of the shafting 
are greatly reduced. With individual drive (Fig. 102), each 
machine is fitted with one or more motors, thus making 

172 



PAR. 187 ] 


SELECTING MOTORS 


173 


it an independent unit. The advantages of the individual 
drive are: (a) Increased production, because slipping of belts 
is eliminated, and speed may always be adjusted to the best 
value for the work. There is also a greater overload capacity. 
Since the labor cost of production is a very large item (often 
50 per cent or more of the total), this advantage is very im¬ 
portant. Experience with the electric drive has shown increases 
of from 10 to 20 per cent in the output of a plant. (6) The loca¬ 
tion of the tool is independent of the line shaft and an arrange- 



Fig. 103.—Arrangement of Motor for a Group Drive. 


ment can therefore be used which will facilitate rapid pro¬ 
duction with a proper routing of the work through the shop, 
(c) New tools can be added and tools easily rearranged. ( d ) 
Line shafts, with their troublesome belts, are eliminated. This 
means greater safety to workmen, a cleaner and better lighted 
workroom, headroom for overhead cranes, and in some cases a 
considerably cheaper building, because less headroom and a 
lighter construction can be used. ( e ) The power losses in 
transmission are less than with shafting, d he total loss, 
including generator, feeders and motors, is from 20 to 30 per 







174 


SELECTING MOTORS 


[CHAP. 11 


cent. The first cost of individual drive is, however, greater 
(about 20 per cent) than group drive. Group drive (Fig. 103) 
is particularly adapted for light machine work, where the in¬ 
dividual machines are small and little or no speed adjustment 
is required. 

188. Choosing Type of Motor. The choice as to the use of 
a.c. or d.c. motors is dependent upon a number of factors besides 
the kind of machine to be driven. This subject is covered 
in paragraphs 162 and 220. The choice as to the type of motor, 
either alternating or direct current, must be based primarily 
upon the starting and running requirements and upon the 
character of the surroundings, as regards dust, inflammable 
material, moisture, etc. Considering load requirements only, 
the classification given below will apply: 

Classification of Motors with Reference to Performance 


Local Requirements. 

Suitable Type 

of Motor. 

A.C. 

DC. 

(1) Approximate constant speed, no load 
to full load. 

Induction motor. 

Shunt motor. 

(2) Semi-constant speed, no load to full 
load. 

Induction motor 
with high rotor 
resistance. 

Compound mo¬ 
tor. 

(3) Speed adjustable, but remaining ap¬ 
proximately constant from no load 
to full load for one adjustment. 

Nothing available 
except in special 
cases for large 
motors. 

Shunt motor 
with field 

rheostat. 

(4) Speed adjustable, and semi-constant 
from no load to full load, for one 
adjustment. 

Nothing available 
except for large 
motors. 

Compound mo¬ 
tor with shunt 
field rheostat. 

(5) Varying speed, decreasing greatly 
with increase of load. 

Induction motor 
with adjustable 
rotor resistance. 

Series motor. 


Detailed classifications of the various types of motors are given 
in paragraphs 150 and 160. With a knowledge of the load re¬ 
quirements, the motor which will be best suited to meet these 











PAR. 189 


SELECTING MOTORS 


175 


requirements can be selected. Open-type motors should be 
used wherever the operating conditions will permit, because 
they are cheaper than enclosed motors of the same horsepower 
rating.* Sometimes dust-proof bearings are required; in other 
cases entirely enclosed motors are necessary where there is 
excessive dust. Semi-enclosed motors should not be placed 
in inaccessible places, as the screens will soon become filled with 
dirt, making the motor practically an enclosed motor, and con¬ 
sequently increasing the heating. Induction motors will with¬ 
stand more severe conditions than d.c. motors because of the 
absence of a commutator, and rarely need to be entirely 
enclosed to protect them from dust. For the same reason, 
induction motors do not need as much protection against mois¬ 
ture and there is, of course, no danger of setting fire to inflam¬ 
mable dust. 

189. Desirable Speeds for Motors. In general, the motor 
speed should be as high as conditions will permit. A high-speed 
motor costs and weighs less than a slow-speed motor and occu¬ 
pies less space. This is shown by the following tabulation: 

Effect of Speed upon Cost and Weight of a Motor 


Shunt wound, 230 volts. 


5 Horsepower. 

10 Horsepower. 

50 Horsepower. 

Speed, 

Cost, 

Weight, 

Speed, 

Cost, 

Weight, 

Speed, 

Cost, 

Weight, 

R.P.M. 

Dollars. 

Pounds. 

R.P.M. 

Dollars. 

Pounds. 

R.P.M. 

Dollars. 

Pounds. 

1800 

143 

265 

1700 

231 

440 

1700 

671 

1210 

1100 

173 

350 

1300 

242 

510 

1100 

704 

1585 

850 

240 

440 

1150 

253 

545 

975 

737 

1895 




850 

286 

645 

565 

990 

2700 



• 

730 

308 

715 





On the other hand, a high-speed motor may be more noisy. 
With a.c. motors only a limited number of speeds are avail¬ 
able, the highest being approximately 1800 r.p.m. for 60 cycles 
and 1500 r.p.m. for 25 cycles.! In any case, a standard speed 
should be selected if possible. Table 25 gives the standard 
speeds commonly used. Unless the motor is directly connected 
♦See paragraph166. t See paragraph 164. 






























176 


SELECTING MOTORS 


[CHAP. 11 


* 

to the machine, there is some freedom of choice. W here belts 
are used, a pulley ratio of more than G to 1 is undesirable. A 
ratio of machine and motor speeds greater than this would 
require counter shafts or idler pulleys to increase the arc of 
contact on the motor pulley. These should be avoided where 
possible. 

190. Effect of Low Efficiency. If the motors operate at low 
efficiency, the cost of power may be considerably increased. 
The low efficiency may result either because the motors are 
not fully loaded or because they are poorly designed. It is not 
always true, however, that the most efficient motor is the best 
to use. A low-efficiency motor would cost less and take up 
less space and the saving in cost may sometimes be invested 
in the business where it will give a greater return. The saving 
in power by the use of a more efficient motor depends upon the 
length of time the motor is used each day and upon the cost of 
power. The high-efficiency motor shows the greatest 'saving 
when the cost of power is high and the period of operation long. 

191. Relation between Load and Motor Rating. Both the 
average and maximum conditions of load must be considered 
when selecting a motor. Sometimes the maximum load require¬ 
ments occur at starting; in other cases, they may represent an 
occasional overload of short duration. It is obvious that the 
motor should be as small as will properly meet the require¬ 
ments, in order to reduce the first cost to a minimum. This 
should not, however, lead one to underestimate the load require¬ 
ments. A motor which is too small would be subjected to 
frequent overloads, would operate at an excessive temperature 
and in the case of a d.c. machine there would probably be 
difficulty in keeping the commutator in good condition. As a 
result, the cost of maintenance and repairs would be excessive. 
If the motor is larger than necessary, besides costing more, 
the operating efficiency would be lower and in the case of induc¬ 
tion motors the power factor would be poor. Motors are so 
designed that they have a good efficiency between one-half 
and full load. Below half load the efficiency falls rapidly. 
As a general rule, the size should be so chosen that the motor 
will operate between three-quarters and full load most of the 
time. Heavy loads of comparatively short duration can be 
taken care of by the overload capacity of the motor.* Manu- 

* See paragraph 165. 


\ 


PAR. 192 ] 


REQUIREMENTS OF MACHINES 


177 


facturers standard sizes and speeds should be chosen, because 
they cost less and repair parts can be more easily secured. 

192. Determining Amount of Load. The amount of power 
required for a given machine can be obtained from the manu¬ 
facturer or, by testing with a temporary motor, or in some 
cases by calculation. W hile occasionally the manufacturers 
are inclined to recommend motors which are too large, the more 
progressive firms have accurate data on the subject and are 
able to make proper recommendations provided the work the 
machine is to be used for is definitely known. The methods 
of calculation given in the following paragraphs will serve 
as an approximate guide to the size of motor required. Wher¬ 
ever possible, however, data based on tests should be used. 
There are a number of tables published which give this informa¬ 
tion.* 


Requirements of Machines 

In selecting a motor, both the starting and running require¬ 
ments of the machine must be considered. The tabulations 
in paragraphs 150, 160 and 162 will assist in choosing the proper 
type of motor when these requirements are known. Below 
are given certain special requirements which have a bearing 
on the selection. 

193. Machine Tools. Group drive is generally preferable 
for bench and speed lathes and moderate size engine lathes. 
Heavy lathes are usually driven from individual motors. Other 
machines which are generally group driven include automatic 
screw machines, sensitive drills, vertical and radial drilling 
machines, boring machines, grinders, shapers, slotters and 
milling machines. Individual drive would be used only for 
very large sizes of these machines or where they are isolated 
from the other tools. Large planers are best driven by in¬ 
dividual motors. Where individual drive is used for lathes, 
drills, etc., the shunt-wound motor is best, because wide speed 
adjustment can be secured by a field rheostat. If induction 
motors are used they would be of the squirrel-cage type and the 
speed adjustment would be made by cone pulleys or by a set 
of gears contained in a “ gear box.” For driving large planers, 
a motor which reverses with each stroke of the table is now 

* See Standard Handbook for Electrical Engineers and Proceedings of the 
American Society of Mechanical Engineers. - - 


178 


SELECTING MOTORS 


[CHAP. 11 


being used extensively. The motor is compound wound and 
specially designed for this service. The controller is arranged 
to permit independent adjustment of the speed of the cutting 
and return strokes over a wide range. Motors for machine 
tools are usually rated on a two-hour basis, since there are fre¬ 
quent periods of shut-down or light-load operation. The 
motor would in general be selected to take care of the average 
load, since the overload capacity of the motor is sufficient for 
the maximum load unless it is of long duration. In every case, 
care must be taken that the motor selected is large enough to 
properly start the machine under the most severe conditions. 
The sizes of motors on machines used by piece workers would 
in general have to be larger than on those where day labor is 
used. The horsepower required for machine tools can be 
approximately determined by the following formulas: 

Lathes, planers, etc. 

Hp. = cubic inches removed XK. . . . (1) 

The cubic inches of metal removed are found by multiplying 
together the depth of cut and the feed, expressed in fractions 
of an inch per revolution or stroke and multiplying by twelve 
times the cutting speed in feet per minute. For a lathe, if 
the speed, in revolutions per minute, is known the cutting speed 
can be found by multiplying the diameter of the work in inches 
by the revolutions per minute and dividing the result by 3.82. 
The value of K is given by the tabulation below: 


Metal. 

A\* 

Horsepower per Cubic Inch 
of Metal per Minute. 

Cast iron. 

0.3-0.5 

Wrought iron. 

0.6 

Machinery steel. 

0.6 

Steel (0.50% carbon). 

1-1.25 

Brass and similar alloys. 

0.2-0.3 


* Am. Soc. Mech. Engr’s, Vol. 32, p. 199. 


Example 1 . Cutting speed CO ft. per minute, ^-in. feed, J-in. 
depth of cut. 

Cubic inches per minute Xi X60 X12 =11. 














PAR, 194 ] REQUIREMENTS OF MACHINES 


179 


Por machinery steel: 

Hp. =11 X0.6 =6.6 Hp. 

Drills. The cubic inches of metal removed per minute are 
found by the formula: 

Cubic inches =0.7854 Xd 2 /.(2) 

Where d = diameter of drill in inches; 

/ = feed in inches per minute. 

The values of K for drills are about double those given above. 

Example 2. For a 2-in. drill working in cast iron, with a feed of 1.5 
in. per minute, we would have: 

Cu.in. =0.7854 X2 2 X1.5 

= 4.7 cu.in. per min. 

From equation (1) 

Hp. =4.7 X0.5 X2 
= 4.7 hp. 

The values of K given do not include the friction of the 
machine. Usually this is not more than 3 per cent of the load, 
so it can be neglected in determining the size of motor required. 
For small tools, such as sensitive drills, the friction load is 
a large proportion of the total load. Wherever possible it is 
best to determine the horsepower required by means of a test. 
In the absence of this, published data,* or information from 
the manufacturers can be used. 

194. Wood-working Machinery. For most wood-working 
machinery, the starting load is light and the machine must 
operate at a constant speed. The ordinary types of induction 
motors are therefore well adapted to this service and are generally 
used. They are more satisfactory than d.c. motors because 
of the absence of a commutator and their large overload capacity. 
For most machines, standard squirrel-cage motors are satis¬ 
factory. Large circular saws and band saws for sawing logs, 
large planers and matchers require a large starting torque 
and are subject to heavy fluctuations in load. For this service 
slip-ring motors with resistance placed permanently in the cir¬ 
cuit, or squirrel-cage motors with a large slip, are used. 

* An excellent list of this kind is given in a publication of the Westinghouse 
Electric and Mfg. Co. entitled “ Electric Motors for Machine Tools.” 


180 


SELECTING MOTORS 


[chap. 11 




195. Pumps. The 



< 


LJ 



Fig. 104. — Diagram 
Showing Head on a 
Centrifugal Pump. 

A = suction lift, or the 
vertical distance in feet 
between centre of pump 
and level of water at 
supply. 

B —discharge head, or 
the vertical distance in 
feet from centre of pump 
to centre of discharge 
outlet. 

Friction head is the 
head in feet, correspond¬ 
ing to friction in both 
suction and discharge 
pipes. If these pipes are 
long, the friction head 
cannot [be neglected. 
With long-radius elbows, 
each quarter turn adds 
about 1 foot to the head. 

Total head =suction 
lift+discharge head -f- 
friction head. 


service which a motor has to perform 
depends first upon the kind of pump 
used. Reciprocating or plunger pumps 
require a heavy starting torque unless 
the pressure on the pump is relieved with 
a by-pass. Only small pumps are, ac¬ 
cordingly, started under pressure. Usu¬ 
ally a squirrel-cage induction motor or 
a shunt motor is employed. Where it 
is desired to change the quantity of liquid 
discharged by the pump, the speed is 
varied, by means of a field rheostat, in 
the case of d.c. motors, and by the use 
of a slip-ring motor where alternating 
current is used. Centrifugal pumps 
have to be treated somewhat differently 
because of their peculiar action under 
different heads. With a reciprocating 
pump operating at constant speed, an 
increase in the resistance against which 
the pump is working increases the pres¬ 
sure, since the volume discharged keeps 
nearly the same. Hence the load on the 
motor increases. With a centrifugal 
pump, on the other hand, an increase 
in the resistance reduces the load. If 
the head is reduced on a reciprocating 
pump, the power required is less; with 
a centrifugal pump, however, a reduction 
of head increases the volume of water 
delivered and the load increases. Some 
centrifugal pumps are so designed that 
the increase in load at the minimum 
head is only about 25 per cent, and this 
can be taken care of by the overload 
capacity of the motor. In others, how¬ 
ever, the load is considerably greater 
than this, so this point must be in¬ 
vestigated carefully when selecting a 
motor. Ifj the speed of a centrifugal 
pump is reduced below normal, the 




























































PAR. 196] REQUIREMENTS OF MACHINES 


181 


quantity of water discharged is reduced. Even the slightly 
lower speed of a shunt motor when first started (with windings 
cold) is sufficient to affect the output considerably. For this 
reason a field rheostat frequently is provided so that the speed 
can be maintained normal at all times. The starting charac¬ 
teristics of centrifugal pumps should also be considered. In 
starting, the discharge valve is usually entirely closed to make 
the starting load as light as possible. Under these conditions, 
the torque is from 15 to 25 per cent of full-load torque and drops 
slightly as soon as the pump turns over. The pump casing 
is filled with water, which is churned round when the motor 
is up to speed, and this produces about half load on the pump 
even with the valve closed. Shunt motors, or either squirrel- 
cage or slip-ring induction motors, are suitable for driving 
pumps. Reciprocating pumps are generally geared or belted 
to the motor, while centrifugal pumps, because of their high 
speeds, can generally be direct connected to advantage. The 
horsepower of a pump can be calculated from the formula 


Hp. 


Total head Xgallons per minute 
3950 Xefficiency of pump 


The total head includes the suction lift, discharge head, and 
friction head. An explanation of these quantities is given 
in Fig. 104. For reciprocating pumps the efficiency is from 50 
to 80 per cent depending upon the size and working condition 
of the pump. For centrifugal pumps, the efficiency is from 50 
to 70 per cent. 

Example. Determine the horsepower required to drive a centrif¬ 
ugal pump delivering 100 gals, per minute, with a suction head of 
15 ft. and a discharge head of 50 ft. There is one elbow in the 
suction pipe and two elbows in the discharge line. The length of the 
line is short, and hence friction can be neglected. Allow 1 ft. loss 
for each elbow. 

Total head =15+50+ 1+1+1 
= 68 ft., 


Hp. = 


68 X100 

3950 X0.50 
= 3.44 hp. 


196. Blowers and Fans. Centrifugal fans have the inlet at 
the centre and discharge through an opening near the outside 
circumference. Propeller or disk fans move the air in a direc. 




182 


SELECTING MOTORS 


[CHAP. 11 


tion parallel to the shaft, similar to the propeller of a ship. 
Blowers include positive-pressure rotary blowers, which create 
a pressure by direct compression. (For example, the Root, 
blower.) The horsepower of any fan or blower increases greatly 
with the speed.* The horsepower of a centrifugal fan at con¬ 
stant speed, in general, decreases as the area of the discharge 
opening is decreased, and with opening entirely closed is only 
about 20 per cent of the power required with full opening. 
The horsepower for a propeller fan increases as the area of the 
discharge opening is decreased, being about double when the 
opening is entirely closed. With a positive-pressure blower, 
the horsepower increases very rapidly as the area of the dis¬ 
charge opening is decreased. With centrifugal fans, therefore, 
the discharge is closed to reduce the load, while with the other 
two types the discharge is left wide open. For centrifugal fans 
and pressure blowers, shunt motors are used for d.c. systems. 
For propeller fans either shunt or series motors can be used, 
the latter being better. With a.c. systems, standard squirrel- 
cage motors are generally used. Fans and blowers frequently 
must be capable of speed adjustment. With d.c. motors, this 
is accomplished by field or armature rheostats or both, and 
with induction motors by means of resistance in the rotor 
circuit. Motors may be direct connected to centrifugal fans 
and to small propeller fans. For blowers and large-sized fans, 
however, the speeds are so low that a belt or chain drive is 
preferable. 

197. Compressors. The load on air compressors is generally 
intermittent. In some cases the machine is started and stopped 
as the demand for air changes, but generally the compressor is 
run continuously and an unloading valve is used to reduce the 
output of the compressor as required. When the compressor 
is started and stopped, the unloading valve is used to reduce 
the starting load. The starting conditions are therefore not 
severe in either case. . Flywheels are used to equalize the load. 

198. Elevators. These machines require a heavy starting 
torque and a constant running speed. One of the great problems 
in high-speed elevator work is to make accurate landings. This 
is very satisfactorily accomplished with d.c. motors by making 
them act as generators feeding into a resistance. With a.c. 
motors the control is not as satisfactory, because a friction brake 

* Proportional to the cube of the speed. 


PAR. 199] 


REQUIREMENTS OF MACHINES 


183 


must be depended upon to bring the car to rest. The d.c. 
motors used have a heavy series winding which is employed in 
starting and is cut out when running, so the machine operates 
as a shunt motor. Slip-ring induction motors are generally 
used with a.c. systems. For high-speed service a two-speed 
slip-ring motor is used. Two separate windings are provided, 
one high-speed for starting and running and the other low-speed 
for stopping. By this means, the motor can be made to act as 
a generator until the elevator has reached one-third or one- 
fifth normal speed, after which a friction brake must be used to 
bring the car to rest. 

199. Hoists. These require a large starting torque. The 
running speed may be variable to suit the load. D.c. motors 
for small hoists are generally series wound, although sometimes 
compound motors are used. With alternating current, slip¬ 
ring motors are employed. For very large hoists, d.c. motors 
operated from a special motor generator set with a heavy 
flywheel are used. The horsepower required to hoist a load 
can be determined from the formula: 

_Load in pounds X speed in feet per minute 

33,000 Xefficiency of hoist 

The efficiency is from 60 to 80 per cent, the latter figure being 
for large, well-designed machines. 

Example. Required the horsepower necessary to operate a hoist 
lifting 6000 lbs. at a speed of 30 ft. per minute. 

6000 X30 

Hp. =- 

33,000 X0.60 

= 9.1 hp. 

200. Cranes. The starting torque of cranes is high and the 
running speed may vary with the load. With d.c. systems, 
the series motor is therefore used, since it tends to slow down 
with heavy loads, and thus relieve the power house of heavy 
current fluctuations. Compound motors* are also used. With 
a.c. systems the slip-ring type of induction motor is best. 
The squirrel-cage type with large slip simplifies the control 
devices and is used for small installations. It is usually dif¬ 
ficult to estimate the power requirements, so that the recom¬ 
mendations of the manufacturer must generally be taken. 

* Series-shunt type. See paragraph 148. 




184 


SELECTING MOTORS 


[chap. 11 


201. Steel Mills. Motors for these places must have high 
overload capacity and great mechanical strength and must 
frequently operate under very severe conditions as regards 
dust and dirt. A.c. motors are used almost exclusively for steel 
mills. For motors driving the main rolls, the slip-ring type 
is used. 

202. Cement Mills. Only induction motors are suitable for 
cement mills because of the dust. The bearings should be made 
dust proof. Slip-ring motors are used on ball and tube mills 
and on crushers which require a large starting torque. They 
are also used on kilns and dryers when speed adjustment is 
required. For the other machines, standard squirrel-cage 
motors are satisfactory. The dust causes rapid deterioration 
of belts, and therefore direct connection is preferable wherever 
possible. A flexible coupling should be used to reduce the 
shocks on the motor. 

203. Tanneries. Either the shunt-wound, d.c. motor or 
the squirrel-cage induction motor has suitable operating char¬ 
acteristics for driving machinery in tanneries. In some parts 
of the process, however, the operating conditions are very 
severe owing to the presence of moisture and acid fumes. For 
this reason induction motors are best adapted for the service 
and are generally used. The windings are specially treated to 
withstand the acid fumes. Switches and fuses are enclosed 
in moisture-proof boxes. Group driving is commonly employed, 
although individual drive is favored for some of the machines. 
Group drive is best adapted for staking, rolling and glazing 
machines, for the tanning and washing drums and for shaving 
and shanking machines. Individual drive is best for belt- 
knife splitters, fleshing and unhairing machines, color drums, 
exhaust fans, pumps, etc. 

204. Textile Mills. As a rule, the loads are steady and 
extremely close speed regulation is required. The starting 
loads are not severe. There is always more or less lint in the 
air and therefore the motors have to be specially protected. 
Squirrel-cage induction motors are generally used. These 
motors are especially designed to give very close speed regu¬ 
lation and a high efficiency and power factor under working 
conditions. Because of the lint, all air ducts in the motors are 
eliminated and the bearings are made dust tight. Enclosed 
motors are not used generally, but the rotor is made without 


PAR. 205 ] REQUIREMENTS OF MACHINES 


185 


any projections to catch the lint. There is a tendency at 
present to use individual motor drive, particularly for looms 
and spinning frames. Overhead shafting and belts are objec¬ 
tionable because they are likely to cause damage to the goods 
from dirt, oil, etc. It is also difficult to maintain constant speed 
due to slipping of belts. 

206. Group drives require constant-speed motors; either 
shunt-wound d.c. or squirrel-cage induction motors. If the 
amount of shafting is large, requiring a large-size motor as com¬ 
pared with the generator, the slip-ring type may be used to 
reduce the starting current. Where a motor is used to drive 
a large amount of shafting to supply, for example, several 
floors in a mill, a synchronous motor is sometimes used to give 
a better power factor. The size of motor required for a group 
drive is less than the total horsepower used by the machines 
driven by it, because they do not all require full load at the same 
time. The approximate size of motor can be determined by 
adding together the horsepower required for each tool and mul¬ 
tiplying this sum by 0.30 or 0.40. To this should be added the 
friction of the line shaft, which is approximately 1 hp. for 
10 to 15 ft. of main shaft. This allows for friction of counter¬ 
shafts and idler pulleys. Where there is one machine much 
larger than the others, this should be included in the total 
at its actual horsepower requirements and not reduced to 
30 or 40 per cent as above described. Wherever possible, how¬ 
ever, the power requirements should be determined by a test 
with a temporary motor. 

206. Selection of Control Equipment.* This is determined to 
a large extent by the character of work the motor is to perform. 
As a rule, starters and control switches should be entirely en¬ 
closed to protect the workman and keep the working parts in good 
condition. Where a motor may be subject to frequent over¬ 
loads as in cranes, hoists, etc., circuit breakers should be used to 
save the time and expense of replacing blown fuses. For small 
motors, the circuit breaker may be combined with the starter 
(overload release). When the motor is started only occa¬ 
sionally the face-plate type is satisfactory. For tools which are 
started frequently the drum-type controller should be used. 
The controller should be so located that it is within convenient 
reach of the workman when he is in position to operate the tool, 

* Refer to Chapter 10, 


186 


SELECTING MOTORS 


[chap. 11 


9 

207. Methods of Connecting Motor to Load. The motor 

may be directly connected to the load or it may be driven 
through belts, gears, chains, or ropes. The direct drive is most 
satisfactory where the speed of the machine is high enough to 
give ail economical speed for the motor. Belt drives, using 
leather, canvas or rubber belts, are employed where there must 
be a difference in speed between motor and load, and where 
the distance between motor and driven shaft is considerable. 
The belt drive is quiet, and is suitable for transmitting large 
amounts of power, provided the distance between shafts is 
sufficient. It has the further advantage of flexibility; thus 
the shocks to motor or machine are not as severe as with the 
positive drives. There is, however, a certain amount of slip 
(from 2 to 4 per cent). Gear drives are used where the distance 
between shafts is too small to allow the use of belts, and where 
a positive drive is required. The drive is usually more noisy 
than a belt, but it has the advantage of saving in space because 
the motor can be placed close to the machine. The loss in 
power with gears varies from 2 to 10 per cent. Chain drives 
(Fig. 106) are used where the distance between shafts is too short 
for belts and too long for connection by gears and where a 
positive drive is required. The pulleys are toothed somewhat 
like gear wheels, and are connected by a specially designed 
chain. As in the gear drive, there is no slip, but the chain 
is somewhat more noisy than a belt. The chain drive can 
transmit large amounts of power at low speeds and is not 
seriously affected by moisture, oil, or grease. It has the further 
advantage that stretching, due to use, does not cause slipping 
as with belts. Rope drives use grooved pulleys and steel 
or fibre ropes. They are used where power must be transmitted 
comparatively long distances, for example, between two build¬ 
ings, and also where the transmission is made at an angle or 
in a vertical direction. Rope drives cost less than belt drives, 
but have certain disadvantages, due to the difficulties in 
equalizing the load carried on the various ropes. 

208. Belt Drives. Leather belts are most commonly used 
and are very satisfactory where they can be kept dry and free 
from oil. Two thicknesses of leather belts are commonly 
used: single (about \ in. thick) and double (about f in. thick). 
Three- and four-ply belts are also used to some extent (for 
heavy service). Single belts should be used on pulleys less than 


par. 208 ] 


MOTOR DRIVES 


187 


12 in. in diameter. The size of belt required to transmit a 
given horsepower can be found by the aid of Table 27. 

Example 1 . Required the width of belt necessary to transmit 10 hp. 
with pulleys each G in. in diameter running at 1400 r.p.m. A single 
belt would be used. From Table 27, we find that a belt 1 in. wide 
will transmit 2.44 hp. The width of belt is therefore 10 4-2.44 =4.1 in. 

A 4-in. belt would therefore be sufficient. 

Example 2. Required the width of belt for a drive to transmit 
50 hp. with one pulley 24 in. in diameter running at 700 r.p.m. and 
the other pulley 48 in. in diameter. A double belt would be used in 
this case. The horsepower for a 1-in. belt would be 


6.08 +(6.83 - 6.08)0.5 =6.46 hp. 


Since the pulleys are of unequal diameter (ratio 2 to 1) the horse¬ 
power is 0.98X6.46=6.32 hp. 

For 50 hp., the width should be 50 4-6.32=7.9 in.; hence an 8-in. 
belt would be used. 



b. Incorrect. 


Correct. 


a. 


Fig. 105.—Arrangement of Belt Drives. 


Rubber belts are made of two or more layers of canvas coated 
with a rubber composition. These belts are specially adapted 
for use in damp places or out of doors. They are very easily 
damaged by oil or grease. Cotton or canvas belts are also used 
to some extent, but for general service they are not as satis¬ 
factory as leather. In calculating a belt drive, the following 
rules may be used: 

Let S = revolutions per minute of driven shaft; 
s = revolutions per minute of driving shaft; 

D = diameter in inches of pulley on driven shaft; 
d = diameter in inches of pulley on driving shaft. 


Then 




D * 


. ( 2 ) 



( 4 ) 






188 


SELECTING MOTORS 


[chap. 11 


Wherever possible, the belts should be run with the slack side 
on top (Fig. 105 a), as this gives a better contact with the smaller 
pulley. Vertical belts should be avoided as much as possible. 
The distance between shafts should not be less than the fol¬ 
lowing:* 


Approximate Pulley 
Ratio. 

Distance between 
Shafts, Feet. 

2-1 

8 

3-1 

10 

4-1 

* 12 

5-1 

15 

6-1 1 

20 


1 This ratio should not be exceeded. 


Excessively small pulleys om motors should be avoided because 
they require a large belt tension, which puts an excessive strain 
on the motor bearing and may cause overheating. The belt 
is also more likely to run off the pulley when the machine is 
being started. Table 26 gives the standard size pulleys recom¬ 
mended by motor manufacturers. Cemented belt joints are 
better than laced joints, especially for high-speed service. 

209. Gear drives have the advantage that there is no slip. 
The speed ratio can be greater than for belting. High-speed 
gears (surface speeds above 600 ft. per minute) are noisy but 
by the use of rawhide or cloth pinions higher speeds (up to 2000 
or 3000 ft. per minute) can be used. The speed of shafts driven 
by gears can be calculated as follows: 

Let T = number of teeth in driven gear; 
t= number of teeth in driving gear; 

S = revolutions per minute of driven gear; 
s = revolutions per minute of driving gear. 

Then 


S = 

sXt 

• • (1) 

SXT 

. . (2) 

rji • • • • 

°r S t . . . 

T = 

sXt 

• • 0) 

4 SXT 


S' ’ ‘ 

or t = -. . . 

s 

. . (4) 


* American Handbook for Electrical Engineer^. 










P4R. 210] 


MOTOR DRIVES 


189 


210. Chain drives, like gears, operate at good efficiency when 
properly designed. They have an advantage over gearing in 
that the work is distributed over a number of teeth (Fig. 106a). 
W ell-designed chains are quieter than gears and have the further 
advantage that the distance between shafts can be varied to 
suit requirements (Fig. 1066). The speed of a shaft driven by 
a chain depends upon the number of teeth in the driving and 



Fig. 106. —Chain Drive. 


driven sprockets, and the calculation can be made by the same 
formulas as for gearing.* 

211. Ordering a Motor. The specification should include 
the following: Kind of motor, voltage, frequency (for alternating 
current), horsepower, whether for continuous or intermittent 
service, speed, kind of machine to be driven, whether open or 
enclosed, details of drive (belt, gear, etc.), whether adjusting 
rails are to be furnished, type of controller. It is also well 
to give the manufacturer as much information as possible 
regarding the service which is to be performed, to enable him 
to recommend a satisfactory motor. It is particularly important 
to give information regarding unusual operating conditions 
such as excessive temperature, dust, dirt, moisture, etc. 


* See paragraph 209. 








PART III. INTERIOR WIRING 


CHAPTER 12 

SYSTEMS OF WIRING 

212. Methods of Power Supply. There are two methods of 
distributing electricity for lighting and power supply. The 
series system, sometimes called the constant-current system 



Fig. 107.—Series System of Distribution. 

Note. —Voltages indicated for lamps include allowance for line drop. Series 
circuits usually have more lamps in one circuit than are shown in diagram. 

as its name indicates, has all the lamps or motors connected in 
series in one circuit (Fig. 107). The current is kept constant 
regardless of the load, and therefore all lamps or motors must 
take the same current. The total voltage of the circuit is the 
sum of the voltages required for each piece of apparatus. The 
total voltage therefore increases as the load is increased by adding 
lamps. With the multiple system, sometimes called the con- 

190 




















































PAR. 212 ] 


METHODS OF POWER SUPPLY 


191 


stant-potential system, the voltage is maintained practically 
constant, whereas the current increases as load is added. The 
total current divides between the various lamps or motors 
which are connected to the system, and hence disconnecting 
a portion of the load has no effect upon the operation of the load 
which remains. The simplest method for multiple distribution 
is the two-wire system shown in Fig. 108. Other systems 
include three-wire and other multivoltage systems, three-phase 
and two-phase systems. All of these are classed as multiple 
systems, since the voltage is maintained nearly constant and the 
lamps or motors are connected directly across the line, The 



series system is especially well adapted for street lighting, 
because a single wire can be run through each street and the 
lamps cut into the circuit at any point (Fig. 107). Also, since 
the current is small (usually 4 to 6.6 amperes), a small size 
wire may be used. The system operates at a high voltage, 
however, and consequently requires careful insulation and is a 
source of danger to anyone who might come in contact vith the 
circuit. It is not possible to carry a large load on a single 
circuit and it operates efficiently only when fully loaded. For 
these reasons, it is not well suited for interior lighting and is 
now very rarely used for that purpose. It is not used for motors 
at the present time* because of the high voltage and the 

* With the exception of a few special European installations. 























192 SYSTEMS OF WIRING [CHAP. 12 

difficulties in regulating the motors with changes in load. Only 
the multiple system will be considered in the chapters following. 

213. Effect of Voltage upon Cost of Wiring. If a given 
amount of power is to be transmitted, it is important to use as 
high a voltage as other conditions will permit. This reduces 
the current required and thus reduces the size of wire neces¬ 
sary. This is apparent from the following: 

Effect of Voltage upon Size of Conductors 


Based on feeder to transmit 100 kw. a distance of 1000 ft. 
with 5 per cent line loss (two-wire system) 


Voltage. 

Amperes. 

Line Loss, 
Volts. 

Line Loss, 

% 

Size of Feeder, 1 
Cir. Mils. 

Percentage of 
Copper, 1 120- 
volt System 
= 100%. 

120 

833 

6 

5 

2,970,000 

100 

240 

417 

12 

5 

744,000 

25 

600 

167 

30 

5 

119,000 

4 

1200 

83.3 

60 

5 

29,700 

1 

2400 

41.7 

120 

5 

7,440 

0.25 


1 In an actual case the feeder would be taken as the nearest standard size, 
which would change the percentage slightly. Correct for d.c. and nearly cor¬ 
rect for a.c. systems. 


The percentage voltage loss in the lines is made the same in 
each case, since it is the 'percentage loss and not the actual 
loss which fixes the limit in each case. In other words, the 
operation of lamps or motors would be as satisfactory on either 
a 120- or a 240-volt system as long as the percentage drop is the 
same. The actual voltage drop in the example given would be 
6 volts for the 120-volt system and 12 volts for the 240-volt 
system. It is apparent that, if the power transmitted is kept 
the same, the current required for the 240-volt system is one- 
half that for 120 volts. Since the allowable drop is doubled 
(the same percentage) the copper required is only one-quarter 
that needed for the 120-volt system. This means that the 
size of feeders for equal percentage loss varies inversely as the 
square of the voltage. This statement is correct for d.c. cir- 

















PAR. 214 ] 


D.C. SYSTEMS 


193 


emits and is nearly correct for a.c. circuits. The tabulation 
above takes into account voltage loss only. Other factors enter 
into this question, such as the greater cost of the high-voltage 
apparatus, the danger to users of the power, more expensive 
maintenance, etc., so that the voltage used must frequently 
be a compromise.* 

214. The Two-wire System. The arrangement of lamps 
or motors is shown in Fig. 109. f This system may be used for 
supplying either direct or alternating current and when used 
for the latter is generally called a single-phase system. The 
two-wire system is very simple, but with direct current only 
one voltage can be supplied. This is a disadvantage when 



Fig. 109.—Two-wire System. 


operating both lamps and motors. Lamp voltages are limited 
to 240 volts, while for motor circuits a higher voltage is fre¬ 
quently desirable. With single-phase systems, the voltage can 
be changed by means of transformers, but single-phase motors 
are not satisfactory except in small sizes. 

215. The Three-wire System. In order to use a high voltage 
for transmitting the energy and at the same time to employ 
low-voltage lamps or other devices, a series arrangement as 
illustrated in Fig. 110a might be used, provided the two lamps 
in a branch circuit each require the same current. If lamp 
(1) is extinguished or burns out, lamp (2) is also extinguished. 
If the junctions between the lamps are connected together 

* See paragraph 220. 

t In this figure and those illustrating the other systems, the field connections, 
switches, circuit breakers, etc., are omitted, as they do not enter into the dis¬ 
cussion and would only complicate the diagrams. See Table 47 in Appendix A 
for explanations of symbols used. 



























194 


SYSTEMS OF WIRING 


[chap. 12 


(line B, Fig. 1106) a circuit is still maintained through lamp 
(2) even when (1) is extinguished. This condition is shown in 


+ 


4 Amperes 





+ 


4 Amperes 

a 


4 Amperes _>. 


A 



11 

1 A. 

1 

1 A. 

1 

1 A. 1 A 

i 1 

1 “ 

X 

S i $ 




Fig. 110.—Development of the Three-wire System. 

In (c) it will be noted that each lamp on -f side takes 1.2 amperes and on — 
side 0.9 ampere, although they are all rated at 1 ampere each. This would 
cause the lamps on the -j- side to burn out after a short time. Values of voltage 
and current given are only approximate. 


Fig. 110c. It is apparent that there are now four lamps on the 
negative side of the circuit in series with three lamps on the 


























































PAR. 215 ] 


D.C. SYSTEMS 


195 


positive side. Since the same total current must flow in the 
positive and negative lines, it is apparent that the voltage of 
the lamps on the positive side would be above and on the nega¬ 
tive side below 120 volts. This would cause the lamps on the 


10 Lamps 



a- BALANCED LOAD 

-f- 10 Amp. -v 



l) - UNBALANCED LOAD 



side having the smaller number to burn above normal candle- 
power, and those on the other side below candlepower. In 
fact, with only a small number of lamps on one side, there is 
always danger of burning out these lamps. The arrangement 




































































) 


196 SYSTEMS OF WIRING [CHAP. 12 

shown in Fig. 1106 and c is therefore not practical, and that 
shown in Fig. 110a is not allowed by the National Electric 
Code. If the wire B is kept at a potential midway between 
A and C (120 volts in the example), then lamps may be added or 
removed from either side and the other lamps will not be 
affected. Such an arrangement is called a three-wire system 
(Fig. 111). The middle wire, called the neutral, is kept at a 
voltage one-half that between the outside wires by using two 
generators as shown in Fig. Ill, by a balancer set, a three-wire 
generator or by other methods which cannot be described 
here. Usually the attempt is made to have equal loads on the 
two sides, giving a balanced system (Fig. Ilia). Under 
this condition, the currents in the positive and negative wires 
are the same and no current flows in the neutral. If the load 
is not the same on the two sides, the currents in the positive 
and negative wires are not alike, giving an unbalanced system. 
The neutral then carries a current which is equal to the dif¬ 
ference between the currents in the outside wires (Figs. 1116 
and c). In the example (Fig. 1116) there are ten lamps on 
the positive side which may be considered to be in series with 
ten of the lamps on the negative. The current for these 
lamps flows directly from the positive to the negative terminal 
of the system. The remainder of the lamps on the negative 
side must be supplied through the neutral and the negative 
line. The direction of the current flowing in the neutral reverses 
when the heavier load is on the positive side (Fig. 111c). The 
load on the positive and negative generators is the same as the 
load on the corresponding side of the system as is shown by the 
diagrams. The amount of unbalancing is usually expressed 
in per cent of the total load. 

Example. For Fig. 1116, we have: 

Load on positive side 10 X120 =1200 watts 

Load on negative side 30 X120 =3600 watts 

Total load =4800 watts 

Difference in load =2400 watts 

TT , , . 2400X100 

Unbalancing -=50 per cent 

4800 

Usually the unbalancing will not exceed 10 per cent. When 
motors are operated from a three-wire system, they would 




PAR. 216 ] 


D.C. SYSTEMS 


197 


in general be connected between the outside wires (as shown by 
the dotted lines in Fig. 111a), so as to use the higher voltage. 
The advantage of the three-wire system is that the load is in 
effect transmitted at 240 volts instead of 120 volts, while at the 
same time it is possible to use 120-volt lamps, which have some 
advantages over 240-volt lamps. Consequently the size of 
the feeders can be greatly reduced.* Three-wire systems 
may be either direct or alternating current, although the former 
is somewhat more common. The d.c., 240-120-volt, three- 
wire system, sometimes called the Edison system, is used in the 
business sections of many large cities for distributing elec¬ 
tricity from central stations. The d.c. system is also used in 
many isolated plants, particularly for office buildings, etc. 
A.c., three-wire systems are used to a considerable extent for 
distributing electricity from central stations and for lighting 
circuits for isolated plants. 

216. The Three-wire Convertible System. It is apparent 
from a study of Fig. Ill that if the positive and negative 


Two-Throw 



b a 

Fig. 112.—Three-wire Convertible System. 


a. Connections when operating three-wire. b. Connections when operating 
two-wire. It is important, in the case of arc lamps and Cooper Hewitt lamps, 
that they shall all be connected on the side of the circuit'which does not reverse 
polarity when operating two-wire. 

wires of a three-wire feeder are connected together and the sys¬ 
tem supplied with the voltage to neutral (120 volts in this case), 
the lamps may be operated as a plain two-wire system (Fig. 
112). This arrangement is sometimes used where the system 
is intended to be operated either from a two-wire isolated plant 
or a three-wire central station service. It is apparent that when 
this system is operated two-wire, the neutral must carry the 
sum of the currents in the other two wires. The cross-section 
of the neutral is therefore made double the size of one of the 

* See paragraph 213. 






























198 


[chap. 12 


SYSTEMS OF WIRING 

outside wires. If the feeder is calculated to give the proper 
drop when operating as a two-wire system, it would require 
just as much copper as a regular two-wire system of the same 
voltage. The cost, however, would be greater, since all the 
switches and panel boards must be three-wire and since 
the cost of three wires is more than the cost of the same 
cross-section of copper when combined into two wires. When 
such a system is operated three-wire, the percentage drop will 
be one-half as much as when operated as a two-wire system.* 
If arc lamps are used on this system, they must all be connected 
between one of the outside wires and the neutral unless a revers¬ 
ing switch is provided. This is to prevent the reversal of 
polarity which occurs when the system is changed from 
three-wire to two-wire. Motors used on such a system may be 
connected between the neutral and either outside wire, since 
a reversal of polarity would not affect them. The result is, 
however, that 115-volt, instead of 230-volt motors must be 
used, with the resulting increase in copper for the branch 
circuits. There is in fact no justification for adopting a system 
of this kind for a new installation (unless it is very small), 
since there are at least two very satisfactory methods for 
operating a three-wire, d.c. system from an isolated plant. 
The cost of a convertible system is much greater than a three- 
wire system, and is considerably more than a plain two-wire 
system. There are no attendant advantages, since a 240- 
120-volt, three-wire system allows the use of 120-volt lamps 
and 240-volt motors, thus giving the most desirable arrange¬ 
ment for each class of service. 

217. The Three-phase System. This is an a.c. system which 
has three equal phase voltages and uses either three or four 
wires. The three-wire, three-phase system (Fig. 113) has 
equal voltages between any two wires of the circuit. Lamps 
are connected between two of the line wires, and motors (shown 
by dotted lines) are connected to all three line wires. Usually 
the lamp loads are distributed equally across the three phases 
so as to give a balanced load. Motors would always give a 
balanced load. If the load is balanced, the current in each 
line wire is 

. Total watts 

Amneres = —-... . 

Voltage between wiresXI.73Xpower factor' 

* See paragraph 318. 



PAR. 217 ] 


A.C. SYSTEMS 


199 


Detailed methods of calculating the currents in the various 
wires are given in paragraph 328. The four-wire, three-phase 
system (Fig. 114) has equal voltages between the three line 
wires, and also employs a fourth wire called a neutral. The 
relation between the neutral and phase voltages is: 

(a) Voltage between neutral and any outside wire is 0.577 
times voltage between outside wires. 



tt-BALANCED LOAD 



6- UNBALANCED LOAD 

Fig. 113.—Three-phase System. 

(b) Voltage between outside wires is 1.73 times voltage 
between neutral and outside wires. 

The relations given above are definitely fixed, so that the volt¬ 
age to neutral cannot be changed without changing the voltage 
between line wires. Lamps are connected between neutral and 
outside wires. Motors are connected to the three outside 
wires and always produce a balanced load. When there is a 
balanced load (Fig. 114a) the current in all line wires is the same 
and no current flows in the neutral.* With an unbalanced 

* See paragraph 328. 










































200 


SYSTEMS OF WIRING 


[chap. 12 


load, the currents in the three line wires will be different and a 
balancing current flows in the neutral. The advantage of the 
four-wire system over the three-wire system is that the motors 
may be operated at a higher voltage than the lamps, thus saving 
in the cost of the feeders, in spite of the additional cost of run- 


1 20 Amp. 



a -BALANCED LOAD 

10 Amp. 



b -UNBALANCED LOAD 

Fig. 114.—Three-phase, Four-wire System. 


mng four wires instead of three. For this reason, the four- 
wire system is used very commonly for distributing alternating 
current from central stations when both motors and lights 
are to be supplied. The three-wire, three-phase system is used 
principally for power supply either from a central station or an 













































PAR. 218 ] 


A.C. SYSTEMS 


201 


isolated plant and is used in some cases for lighting circuits 
only.* 

218. The Two-phase System. This is an a.c. system which 
has two equal-phase voltages and uses either three or four 
wires. The four-wire, two-phase system is illustrated in Fig. 
115. Two wires are used for each phase. There is usually 
no electrical connection between the two phases, although they 
may be supplied from a single generator or transformer. Lamps 
are connected to each phase as shown in Fig. 115. Motors are 
connected to all four wires. Since the two phases are indepen¬ 



dent, the circuits can each be calculated like a single-phase 
circuit. By combining one wire from each phase a three- 
wire, two-phase system is produced (Fig. 116). Between 
each phase wire and the common wire the voltage is the same, 
while between phase wires (1 and 2, Fig. 116) the voltage is 
1.41 times the voltage to the common wire. Lamps would be 
connected between the phase wires and the common wire. 
Motors would be connected to all three wires. For a balanced 
load and a four-wire system, the current in each wire is 

Total watts 

\ m dotps == —*—. .. - - — ---- 

1 ’ Voltage between wiresX2Xpower factor' 

This formula can also be used to calculate the current in the 
outside wires of a balanced three-wire system. The current 

* A notable example of this is the Pennsylvania R.R. passenger terminal 
in New York City, where a 240-volt, three-wire, three-phase system is used for 
supplying the lighting. 

















202 


SYSTEMS OF WIRING 


[CHAP. 12 


in the common wire is 1.41 times the current in one of the out¬ 
side wires.* The three-wire system requires less copper than 
the four-wire system,! but the voltage drop in the common 
wire has a disturbing effect upon both the phase voltages. The 
system is used more commonly for motor service. 




b - UNBALANCED LOAD 

Fig. 116. —Two-pliase, Three-wire System. 

219. Comparison of Systems. The relative amount of 
copper required to transmit a given load when using the 
various systems described in the previous paragraphs is shown 
by the Table on p. 203. This comparison is based on 
balanced loads and the same percentage drop is assumed 
in each case. The voltage at the load is assumed the 
same in each case. The values given are correct for direct 
current. For alternating current, especially at low power 
factors, the percentage drop is somewhat larger. It is not 
possible, in an actual installation, to obtain all the saving 
indicated by this table because the total installed cost for a 

* See paragraph 329 for methods of calculating current with unbalanced 
loads. 

f See paragraph 219. 




































PAR. 219 ] COMPARISON OF SYSTEMS 203 


Comparison of Systems of Distribution on Basis of Copper 

Required 


1 

System. 

Con¬ 

nections. 

Voltage Relations. 

Relative Amount of 
Copper in Per cent. 

Two-wire, d.c. or 
single-phase. 

Fig. 109 

*1 

O 

100 

Three-wire, d.c. 
or single-phase. 

Fig 111. 

TT 

1 -g- 

o 

o 

i-H 

With neutral same 
size as other wires, 
37.5 

With neutral half 
size, 

31.2 

Three-wire con¬ 
vertible, d.c. or 
single-phase. 

Fig. 112 

K 

K 

c 

C 

J 

' % i 

. o 

*• V 1 K 

S «4- >- 

c 

L.I J 

. o 

' 

o 

o 

yri 

— •- 

Neutral double size 
of outside wires, 
100 

Three-phase 
(three wires). 

Fig. 113 

A 

jC-100 V.—& 

75 

Three-phase 
(four wires). 

Fig. 114 

A, 

Neutral same size 
as outside wires, 
33.3 

Two-phase 
' (four wires). 

Fig. 115 

r 

o 

y 0 £-100 

100 

Two-phase* 

(three wires). 

• 

Fig. 116 

> A 

o 

-1® \ 
Kioo \> 


Common 1.41 times 
as large as outside 
wires. 

73 

k > 
















































204 


SYSTEMS OF WIRING 


[CHAP. 12 


single large wire is less than for two small wires containing 
the same amount of copper. 

Example. A two-wire No. 0000 feeder will carry a given load 
at 120 volts with the same percentage drop as a No. 3, three-wire, 
240-120-volt feeder. The copper required is: ■) 

Two-wire system, 2 X212.000 =424,000 c.m. 

Three-wire system, 3 X52,600=157,800 c.m. 

Relative amount of copper, 157,800 -f-424,000 =0.372. 

Cost of No. 0000, two-wire feeder in conduit, $1.30 per foot. 

Cost of No. 3, three-wire feeder in conduit, $.71 per foot. 

It is apparent, however, that there is still a considerable sav¬ 
ing in favor of the three-wire system. 

220. Choice of Systems for Lighting and Power Service. 
The standard voltages for d.c. generators are 125, 250, and 575, 
and for a.c. generators 240, 480, 600, and 2300. The system 
would not necessarily be operated at exactly these values, 
but might be somewhat higher or lower. The actual operating 
voltage would be so chosen as to give the correct voltage on 
the motors. Since incandescent lamps can be obtained for a 
considerable voltage range (105 to 125 volts for example), they 
would be selected to suit the actual voltage conditions as fixed 
by the motor load. It is apparent, from what has been said 
in paragraph 213, that the voltage of the system should be as 
high as other conditions will permit. For d.c. systems the 
voltage is limited to 240 volts where incandescent lamps are 
used. A two-wire, 240-volt' system is much cheaper than a 
120-volt system, but 240-volt lamps are less efficient and more 
expensive. The cost of maintenance of the 240-volt system 
would also be greater because of the higher voltage on switches 
and sockets. A shock from a 240-volt system is also more 
serious. The three-wire, 240-120-volt system costs only 
slightly more than a 240-volt, two-wire system, and has the 
great advantage that 120-volt lamps and other devices may be 
used. Where motors only are to be supplied, 240 or even 
600 volts may be used if the feeders are very long. A voltage 
higher than 600 is not satisfactory for industrial purposes because 
of the danger from shock and the extra cost of the motor con¬ 
trol devices. The only applications of the higher voltages 
are for heavy railway work, where the additional costs are 
justified by savings in feeders and substation apparatus. Be¬ 
cause the voltage is limited to not more than 600 volts for 


par. 220] 


CHOICE OF SYSTEMS 


205 


industrial purposes, direct current is used only where the 
length of the feeders is comparatively short, as in a single build¬ 
ing or a group of buildings located together. It is also used in 
the business districts of a number of large cities. Direct cur¬ 
rent must be used for charging storage batteries and for elec¬ 
trolytic work and it is preferable for cranes and adjustable 
speed motors. When a.c. systems are used the ease with which 
the voltage can be changed by means of transformers makes 
it possible to take full advantage of the saving resulting from 
the use of a high voltage. In such cases the power can be 
transmitted at a high voltage, requiring small-size feeders. 
At the points of use the voltage can then be reduced by means 
of transformers to a low value suitable for lamps and motors. 
The small loss of power in the transformers is more than off¬ 
set by the saving in the feeders. Here again there are certain 
limitations to the voltage which should be used. As the 
voltage is increased the conductors must be better insulated 
and all switches, transformers and other apparatus would 
cost more. In a particular problem, therefore, it may be 
necessary to balance the saving in cost of feeders against the 
additional cost of the other apparatus. In general, it may 
be said that for industrial plants 600 volts will be sufficient 
even for large plants. Sometimes a voltage of 2300 is used, 
but this is seldom necessary unless the plant covers a large area 
and considerable power must be transmitted. This voltage 
is very commonly used by central stations for distributing 
power to customers. Voltages higher than this are generally 
used only for transmission of power in large quantities for long 
distances. The choice of a system is in many cases fixed by the 
available sources of supply. With a central station supply, 
if direct current, the Edison three-wire, 240-120-volt system 
would generally be employed. Lighting services supplied from 
these mains would be three-wire, and power services would 
be two-wire, 240 volts. If an a.c. supply is furnished, trans¬ 
formers would be used either for individual customers or for 
supplying low-voltage secondary mains. For lighting services, 
a single-phase, 120-volt system would be used for small cus¬ 
tomers and a 240-120-volt, three-wire supply for larger instal¬ 
lations. Small motors would be single-phase and would be 
operated from the lighting service. Most companies restrict 
these motors to small sizes because of the serious voltage 


206 


SYSTEMS OF WIRING 


[chap. 12 


fluctuations which they cause. For general motor service 
220-volt, two-phase or three-phase systems are commonly 
used. Sometimes for the supply of a combined load of lamps 
and motors, a 120-208-volt four-wire, three-phase system is used. 
Where the load is supplied from an isolated plant, for either 
an industrial establishment, an office building or similar ser¬ 
vice, there is greater freedom of choice. Even here the pos¬ 
sibility of obtaining an auxiliary supply from a central station 
may have to be considered. The relative size of power and 
lighting loads will have an important bearing upon the selec¬ 
tion of a system when an isolated plant is used. In some 
cases of light manufacturing, particularly if all the work is 
in one building, where the feeders would be short, direct 
current might well be used, employing 120 volts two-wire 
for small systems, and 240 volts three-wire, or possibly two- 
wire, for larger systems. If a two-wire system is used, the 
feeders would be about one-fourth as large for 240 volts as 
for 120 volts; but, on the other hand, the lighting would have 
to be supplied at 240, which would entail somewhat greater 
cost for lamps and maintenance. It is better to operate the 
motors at 240 volts and supply the lights on a 120-240-volt 
three-wire system. By this means, the saving in size of feeders 
is nearly as great as if the entire load were supplied at 240 
volts and the advantage of the lower-voltage lamps is secured. 
The additional power-house equipment required is of small 
cost. For most industrial uses, a.c. induction motors are 
satisfactory, and in some cases are almost necessary, either 
because of the great distances from the power-house or the 
severe operating conditions due to dust, moisture, etc. The 
principal disadvantage is the difficulty of adjusting the speed.* 
A.c. motors are not satisfactory for cranes and elevators, 
owing principally to the difficulty of control, particularly 
when making stops. For this reason d.c. motors are to be 
preferred for high-speed elevators and large cranes. There¬ 
fore, in an office building where the elevator load is usually 
greater than the other motor load and the length of the feeders 
is not great, the d.c. system is preferable. For large build¬ 
ings the three-wire, 240-volt system should be used, the motors 
operating at 240 volts and the lights at 120. Only in small 
buildings should the 120-volt two-wire system be used. If 

* See paragraph 153. 


par. 220] 


CHOICE OF SYSTEMS 


207 


only alternating current is available it would be best to use 
a.c. elevators unless the speed is high (above 300 ft. per minute) 
rather than provide the necessary transforming apparatus. 
For industrial establishments in general, alternating current 
is to be preferred unless the cranes and adjustable-speed tools 
form a large proportion of the total load. For the usual 
industrial plants, the three-phase, three-wire system is most 
commonly used, although two-phase systems are employed to 
some extent. The voltages may be 220, 440 or 550, but at 
present there is a tendency towards the use of 440 volts rather 
than 220 volts, so as to reduce the cost of the feeder system. 
One disadvantage of all a.c. systems is that the voltage drop on 
the wiring is greater than for direct current where motors or arc 
lamps are operated. This is particularly true if open wiring 
is used.* This disadvantage can, however, be largely offset by 
the use of a higher voltage, which is feasible with a.c. circuits. 
A frequency of 60 cycles is generally used because it is better 
than 25 cycles for motors.f In many industrial establishments, 
as for example, railroad shops which are spread over a large 
area, alternating current is used for general power supply. 
The machine tools and cranes which can be operated better 
by direct current are supplied by means of a motor generator set 
or a rotary converter. For such service a 120- or 240-volt, 
two-wire system is generally used. 


* See paragraph 322. 


t See paragraph 164. 


CHAPTER 13 


METHODS OF INSTALLING WIRING 

221. National Electrical Code. All interior wiring should be 

installed in such a way that it will be protected from mechanical 
injury and w T ill be safe as regards fire hazard or danger from 
shock. The apparatus used should be substantially built and 
suited to the surrounding conditions so as to reduce the cost of 
repairs to a minimum. Wherever wiring is installed in a build¬ 
ing upon which fire insurance is issued (which naturally includes 
nearly all cases), it is necessary to conform to the rules of the 
National Electrical Code. This code is issued by the National 
Board of Fire Underwriters and is in effect throughout the 
United States and Canada. The Code gives definite rules for 
the installation of all kinds of wiring and also specifies carefully 
the kind of material (wire, conduit, fuses, etc.) which may be 
installed. This Code is revised every two years. The Na¬ 
tional Board of Fire Underwriters maintains laboratories in 
New York and Chicago where the various fittings and mate¬ 
rials used for wiring are tested. Inspectors are also sent to 
the factories where wire, conduit, etc., are manufactured 
to see that the material is made in accordance with the rules. 
All fittings and materials which pass the tests are “ approved ” 
for installation under the Code rules and are included in the 
List of Electrical Fittings, which is issued by the Board and 
is revised twice a year. Unless the device is given in this list 
it cannot be used where the Code rules, must be followed. Most 
of the material listed, such as conduit, wire, moulding, panel 
boards, etc., is labeled (Fig. 117). This makes it possible 
for the user to determine easily if the material is approved. 
Sockets, receptacles, fuses and similar fittings are not so 
labeled, but approved fittings can be identified by the man¬ 
ufacturer’s catalogue number. Copies of the Code or the List 
of Fittings may be obtained by applying to the Board of Fire 
Underwriters at its New York, Chicago, or Boston offices or 

208 


par. 222] 


RIGID CONDUIT SYSTEMS 


209 


to local inspection departments. The inspection depart¬ 
ment of the Associated Factory Mutual Fire Insurance Com¬ 
panies, with headquarters in Boston, has issued the Code with 
explanatory notes which give more definite instructions regard- 
ing approved methods of installation. This book will be found 
useful for reference. Most of the large cities in the United 
States also have more or less complete 
rules for the installation of electric 
wiring. They are all based upon the 
National Electrical Code and in some 
cases go farther and prohibit the use 
of certain kinds of wiring which are 
allowed by the Code. Where local rules 
exist, they would govern in place of 
the National Electrical Code, and it 
is therefore always necessary, when 
planning an installation, to become 
familiar with these local rules. In 
the following, the rules of the National 
Electrical Code only will be considered 
and the references given in the description of apparatus and 
methods are to the paragraphs in this Code (1915 edition) 
which govern the installation. 

222. Types of Wiring. Interior wiring for lighting or power 
service, at voltages not exceeding 550 volts, may be installed 
in (1) rigid conduit, (2) flexible conduit, (3) armored cable, 
(4) moulding, either metal or wood, (5) concealed, on knobs 
and tubes, and (6) exposed on insulators or cleats. All of these 
methods are approved by the Code, but the use of some of them is 
restricted to special places. 

Rigid Conduit Systems 

223. Description. In this arrangement, a special grade of 
iron conduit or pipe is installed with suitable bends, couplings, 
etc., so as to make a continuous wire-way between the outlets, 
in which the wire may be installed or removed and replaced 
with very little labor. The conduit may be run on the surface 
of walls or ceilings or may be concealed in the walls and floors 
during construction. At each outlet, a suitable outlet box 
is provided to allow for connecting to the wiring. The advan¬ 
tages of a properly installed conduit system are: (1) the wires 


,1/NDERWRI7- £RS ’ 

^laboratories/ 

\NSPECT££j 
CONDUIT 
■ B ‘S - 


Fig. 117.—Label Used 
for Approved Con¬ 
duit. 

Labels similar to this are 
attached to each length of 
conduit. Other approved 
fittings are labeled either on 
the box or on each piece. 


I 




210 METHODS OF INSTALLING WIRING [CHAP. 13 

are thoroughly protected against mechanical injury, (2) danger 
of fire due to short-circuits in the wiring is avoided, (3) the con¬ 
duit may be made water tight so as to protect the wiring 
against excessive moisture, (4) the conduit may be placed in 
a concrete floor or a brick or tile wall during construction of 
the building and the wires installed later, after the rough 
building work is finished, (5) damaged wires may be easily 
replaced at any time without disturbing the conduit system. 
The disadvantage of the system is its high cost. 

224. Applications. A rigid conduit system may be used for 
all classes of service, but it is chiefly used in buildings of fire¬ 
proof construction. It is in fact the only system suitable for 
such buildings, where the wiring is concealed. Flexible con¬ 
duit or armored cable may be used in most cases of this kind, 
but they are not as satisfactory. Rigid conduit is also frequently 
used for circuits run exposed in power houses and industrial 
establishments. For the latter class of service, it is especially 
well suited for the branch circuits leading to the motors and for 
the lighting circuits. In any place where the wiring is exposed 
to possible mechanical injury or excessive moisture, conduit 
systems are very satisfactory. 

225. Construction of Conduit. The conduit used for this sys¬ 
tem must be thick enough to withstand considerable hard usage. 
It must resist nails, hard blows and should not be flattened by 
being walked upon or by being run over by wheelbarrows, etc. 
All this is necessary because the conduit is installed in fire¬ 
proof buildings during the construction of the walls and floors 
and is frequent^ left exposed a long time, before it finally is 
concealed. The thickness of the conduit must also be suf¬ 
ficient to prevent burning a hole in the conduit if a short-cir¬ 
cuit of the wires occurs. There is always the chance of this 
occurring, particularly as all kinds of insulation deteriorate 
with age. The standard conduit (Fig. 118) for rigid systems 
consists of a mild-steel pipe, having the same dimensions, as 
regards thickness of wall, inside bore and threading, as the 
standard wrought-iron pipe used for gas and water pip¬ 
ing (Rule 58).* The sizes are designated by the approx¬ 
imate inside diameters. The act al inside diameters are, 
however, slightly greater than the nominal size given. Thus a 
^-in. conduit has an inside diameter of f in. Table 28 

* This and following references are to rules of the National Electrical Code. 


par. 226 ] 


RIGID CONDUIT SYSTEMS 


211 


gives dimensions of standard conduit. The steel used for 
conduit is softer than that used for ordinary pipe, so 
that it can be bent easily. Conduit is furnished in 10-ft. 
lengths, threaded at each end, with a coupling on one end. 
Nothing smaller than 
|-in. trade size of con¬ 
duit is allowed (Rule 
28a). Any size up to 
6 in. may be obtained, 
although a size larger Fig. 118.—Iron Conduit, 

than 4 in. is seldom 

used. At one time a considerable amount of iron conduit 
having an insulating fibre lining was used. This is not now 
approved by the Code. In the manufacture of iron conduit, 
the pipe is first carefully cleaned of rust or scale and all burrs 
on the inside are removed. In making the best grades of 
conduit, the pipe is then coated inside and out with zinc 
(galvanized) or other protective metallic coating. The inside is 
then covered with an enamel coating which gives a smooth 




Fig. 119.—Conduit Bends. 

a. Installation using standard elbows, b. Installation using conduit bent 
to fit. 

inside surface to assist in pulling in the wires. A somewhat 
cheaper conduit is made in which only an enamel coating is 
used on the inside and outside. The galvanized is best where 
exposed to moisture, cement, etc. 

226. Conduit Bends. Where a turn is made in a conduit 
run, the elbow or bend must have a radius large enough to enable 
the wires to be pulled in without damage. The radius of the 








































































212 


METHODS OF INSTALLING WIRING [CHAP. 13 


curve of the inner edge of any 
3^ ins. (Rule 28 h). The radius 
the size of the -conduit (Table 



Fig. 120.—Bending Conduit with 
Hickey. 

Home-made hickey consisting of 1-inch 
pipe Tee and short length of iron pipe. 
Conduit is held down to floor with the 
foot, and bend made as desired. 

\ 

cold, by hand, by the use of a “ 
the hickey is slipped over the 
then a small bend is made. T 


elbow must not be less than 
must, of course, increase with 
28). Stock bends (Fig. 132) 
may be obtained from the 
manufacturers for all sizes 
of conduit, but these are 
ordinarily not used for f or 
f in.-conduits, as these sizes 
can be easily bent. In ex¬ 
posed work, particularly 
where appearance is impor¬ 
tant, it is better to bend all 
sizes of conduit rather than 
to 4 ise stock bends, because 
of the better appearance of 
the work. This is illustrated 
in Fig. 119. Small sizes of 
conduit may be easily bent 
hickey.” To do this (Fig. 120) 
pipe to the proper point and 
he hickey is then slipped along 



Fig. 121.—Devices for Bending Conduit. 

a. Hickey. A piece of 1-inch pipe is used for a handle, b. Small bending 
machine. 


further and another slight bend is made. This process is 
repeated until the required bend is secured. If a considerable 
bend is made each time the radius will be smaller than if only 
a slight bend is made before moving the hickey. The exact 
manipulation to secure the proper bend at the right place 

































par. 227] 


RIGID CONDUIT SYSTEMS 


213 


cdn be learned only by practice. In here a large number of 
bends are to be made a bending machine (Fig. 1216) would 
be used. Large conduits, even 4 in. or more, may be bent 
cold if a suitable rig is made for doing the work. If only a 
few bends are required, however, the cost of building the 
necessary forms, etc., would be too great, and the conduit 
could better be heated and bent. 

227. Outlets. Wherever there is an outlet, such as a light¬ 
ing fixture or a switch, a suitable outlet box must be provided 



Fig. 122.—Outlet Boxes. 

a. 4-inch square box. b. 4-inch square box with cover for fixture outlet 
and knockouts for gas pipe. c. 4-inch square box with cover for push-button 
switch, d. 31-inch octagonal box. e. 4-inch shallow box. /. Cover with 
bushed hole for drop cord. 


(Rule 28 d). These boxes are usually made of sheet steel, 
although cast-iron boxes are used to some extent, particularly 
in exposed work. Types of steel outlet boxes, suitable for 
concealed wiring, are shown in Fig. 122. These boxes serve 
as a terminal for the conduits and also provide a space for 
making the necessary connections between the circuit wires 
and the fixture or other device which is attached to the outlet 
box. When used with switches, they serve to protect the 
switch parts against injury. They also connect together the 




















































214 


METHODS OF INSTALLING WIRING [CHAP. 13 


entire conduit system so that it can be thoroughly grounded.* 
For small fixture outlets, or where only two of three conduits 
enter the box, a 3j-in. diameter octagonal box is satisfactory 
(Fig. 122d). In some cases 31 or 4-in. round boxes are used, but 
these are not allowed in some localities because of the difficulty 
in making a good connection with the conduit. For switch out¬ 
lets and larger fixtures, a 4-in. square box (Fig. 122 a, b, c ) about 
11 ins. deep is frequently used. A shallow box (Fig. 122e) f or f 
in. deep is used for outlets attached under floors built of terra¬ 
cotta tile or concrete, where the plaster 
is placed directly on the concrete or tile. 
The depth of this box allows only for 
the thickness of the plaster (see Fig. 40). 
Where the plaster is very thin or where 
the outlet must be installed on a finished 
plaster surface, an outlet plate is allowed 
(Rule 28d). This consists of a 4-in. 
diameter plate (similar to Fig. 122e) 
with flanged edges raised about 1-in. 
All outlet boxes (except the shallow 
box, Fig. 122e and outlet plates) are 
provided with covers which enclose the 
box and leave an opening large enough 
only for the switch or fixture. When 
fixtures provided with canopies are used, 
Fig. 123.— Arrange- the cover usually has a 3-in. round 
ment for Switch opening (see Fig. 39). The arrangement 
Outlet in Partition. of a push-button switch outlet is shown 

in Fig. 123. Outlet boxes and plates are 
usually galvanized or sheradized,f although black enamel boxes 
are also used. In steel boxes, the openings for conduit are 
provided by means of knockouts, which consist of steel plugs 
filling the holes and held in place by a small web of metal. A 
blow of a hammer is sufficient to remove the plug and leave 
a hole suitable for the conduit. Outlet boxes are provided 
with a number of knockouts in the sides and bottom, so that 
the conduit can be installed at the desired point. The size 
of the knockouts must of course fit the size of conduit used, 
which is generally \ in. or f in. Knockout plugs must be 

* See paragraph 232. 

t Heated in presence of zinc dust. This gives a zinc coating on the surface. 















































pah. 227] 


RIGID CONDUIT SYSTEMS 


215 




'PLUS'S* 


hk ■•'- 


Fig. 124.—Switch Outlet. 

Three-gang box for three push-button 
switches (flush type), installed between stud- 
ding. Box is fastened to a wooden strip 
nailed to studding. 


removed only where conduit or gas pipe is to be installed 
(Rule 59c). Outlet boxes are made in a large number of sizes 
with covers to fit various types of switches and fixtures. The 
reader is referred to trade 
catalogues for detailed 
information on the various 
types. Where the wires 
leave an exposed end of 
a conduit, as for example 
at a motor, or where the 
wires are to be continued 
as exposed wiring (Fig. 

127), the conduit must 
terminate in a suitable 
bushing (Rule 28 d). This 
must provide a separate 
hole for each wire and 
each hole must be bushed 
with an insulating material 
such as porcelain or moulded composition. There are many 

forms of such fittings on 
the market, one kind being 
shown in Fig. 128. This 
illustrates only two of the 
many styles available. In 
Fig. 224 is shown the use 
of these fittings for motor 
wiring. Fittings of this 
type are also frequently 
required for the ends of the 
conduits behind switch¬ 
boards. In power stations, 
however, where the board 
is subject to expert super¬ 
vision, the ordinary metal 
conduit bushing (para¬ 
graph 228) would gen¬ 
erally be accepted. A 
question of this kind should be taken up with the local inspec¬ 
tion department if there is any doubt as to the requirements. 
In Fig. 129 is shown a method of terminating a conduit for a 




,..U- - 

■ llisr. ■ 

"MX ■* ■■ 

** '■ ^ * ** * ** 

: I 

■h I 






Fig. 125. —Switch Outlet. Ready 
to plaster in. 

Showing wire lath in place over box 
cover. Outlet is for a single push-button 
switch, flush type. 












216 METHODS OF INSTALLING WIRING [CHAP. 13 

circuit entering a building from an overhead line. For exposed 
conduit, sheet-steel outlet boxes are sometimes used, but it is 
more common to employ cast-iron boxes or special fittings 
(such as condulets) which are threaded to receive the conduit. 
These fittings are made in a large variety of styles, suitable for 
switches, cutouts, lamp outlets, etc., and are very convenient 
to use and give a good appearance to the finished installation. 



Fig. 126.—Ceiling Outlet. 

Showing arrangement for a rigid conduit installation where box is located 
on a floor joist. 


While they are somewhat more expensive than ordinary cast- 
iron boxes, the improved appearance and the saving in the cost 
of installation offsets this objection. Examples of these fit¬ 
tings are given in Fig. 130. The fittings are now made in styles 
to suit practically all conditions which may arise. The use 
of these fittings is illustrated in Fig. 131. 

228. Bushings and Locknuts. All ends of conduits must 
be provided with bushings to protect the wire from damage 
(Rule 28e). With condulets and similar fittings the openings 



par. 229 ] 


RIGID CONDUIT SYSTEMS 


217 


are properly rounded so that no additional bushings are re¬ 
quired. In the ordinary sheet steel boxes (Fig. 122), steel bush¬ 
ings must be provided for each conduit end. These bushings 
are shown in Fig. 132. Bushings of this kind would also be 
used for conduit which does not terminate in an outlet box 
and where insulated bushed holes are not required.* Lock¬ 
nuts (Fig. 132) are used to 
make a rigid connection 
between the steel outlet box 
and the conduit. Fig. 41 
shows a conduit installed in 
a steel outlet box. Bushings 
and locknuts are usually 
galvanized or sheradized. 

Couplings (Fig. 132) are used 
to join the lengths of con¬ 
duit and are similar to the 
couplings used for water 
piping. Dimensions of these 
229. Wire. 



Fig. 127.—Method of Protecting 
Wires Leaving Conduit. 


fittings are given in Table 29. 
The wire used in conduit systems must be insu¬ 
lated with rubber and covered with a tough protecting braid. De¬ 
tails of the construction of this wire are given in Chapter 14. 
For single wires smaller than No. 6, a single braid is allowed 
(Rule 26ft). For twin and multiple conductor wires and for single 



Fig. 128.—Fittings for Bushing Ends of Conduit. 

Covers with different numbers of wire holes are manufactured. 



wires, No. 6 and larger, a double braid is required. In locations 
where the wires are subjected to very high temperatures, rubber 
insulation is likely to be damaged and in such cases, if in dry 
locations, slow-burning insulation f is allowed by special per¬ 
mission. Wires larger than No. 8 should be stranded, because 
of the difficulty of pulling large solid wires into conduit and the 

* See paragraph 227. t See paragraph 258. 



























































































































218 METHODS OF INSTALLING WIRING [CHAP. 13 

greater likelihood of the insulation of such wires being damaged 
during the process. The sizes of conduit used for the wires 
must be large enough to enable them to be installed without 

damage. The.size will depend to 
some extent upon the length of the 
run and the number of bends. Not 
more than four right-angle bends or 
their equivalent are allowed in a run 
between two outlets, bends near 
the outlets not being.counted (Rule 
28/t). Table 30 gives the required 
sizes of conduit for various sizes and 
number of wires in a conduit. This 
is based on the use of wire suitable 
for any voltage up to 600 volts. 
Wires for higher voltages have 
thicker insulation and would require 
larger conduits. The sizes given 
in this table are the smallest 
allowed by the Code (Rule 28 i) and would apply, in general, 
to runs having not more than three right-angle bends for small 
wires (No. 10 and smaller) and two bends for larger wires. 



Fig. 129.—Wires Enter¬ 
ing Building through 
Conduit. 

Showing use of weather¬ 
proof bushed fitting at end of 
conduit. (Factory Mutual 
Ins. Co.’s.) 



Fig. 130.—Condulets. 

a. T-fitting used for junction box. b. Outlet for flexible cord pendant fixture, 
c. Push-button switch box. d. Outlet for low ceiling, e. Receptacle. 

If the wire size is the largest specified for a given size of conduit, 
it would be necessary to go to the next larger size of conduit, 
where there are more bends or where the length of run is great. 
An offset bend is especially hard to pull around and should 











































































































par. 230 ] 


RIGID CONDUIT SYSTEMS 


219 



be counted as two right-angle bends. Where the distance 
between outlets is very great, it may be necessary to insert 
a pull-box (Fig. 133) in the conduit run, so that part of the wire 
can be pulled in at a time. The size of pull-box required would 
depend upon the conduit size. Table 31 gives sizes of these 
boxes. The smaller sizes may be made of cast iron, but the 
larger sizes should be sheet steel. It is a mistake to use too 
small a conduit. If this is done, 
the wire may be injured during 
process of pulling in, and may 
cause a short-circuit after the 
system is in operation. Even if 
this does not occur, the cost of 
the additional time required to 
pull in a circuit where the wire 
fits too tightly will often be much 
greater than the difference in cost 
if the next larger size of conduit 
is used. Not more than four two- 
wire circuits, nor more than three 
three-wire circuits are allowed in 
a single conduit except in special 
cases. The same conduit must 
never contain wires of different 
systems (Rule 26p). 

230. Installing Conduit. The 
size of conduit is determined by 
the size and number of wires 
which it must contain, as explained 
in the previous paragraph. Where 
direct current is used each wire 
can be installed in a separate con¬ 
duit if desirable, but with alter¬ 
nating current all the wires of the 

circuit must be installed in the same conduit * (Rule 26p). It is 
well to do this even where direct current is used, if there is 
a chance that the system will be operated by alternating cur¬ 
rent in the future. If brass, fibre or tile ducts are used, the 
wires can be separated, but if this is done the voltage drop is 
increased.! The use of a single conduit for all the wires would 

* See paragraph 325. t See paragraph 322. 


Fig. 131. —Example of Use 
of Condulet Fittings. 

Showing use of L-fittings for 
turning sharp corners. 











220 


METHODS OF INSTALLING WIRING [CHAP. 13 


give a lower cost of installation and would therefore be used 
except where the size of the wires is so great as to require exces¬ 
sive sizes of conduit. * The conduit system must be continuous 
between outlets, so that the wire can be installed after most 
of the mechanical work has been completed, without dis¬ 



ci. Elbows. 


Fig. 132.—Conduit Fittings. 

b. Couplings, c. Bushings, d. Locknuts, e. Pipe straps. 


turbing the conduit. For concealed work in fire-proof buildings 
having hollow tile walls and floors, the conduit and outlet boxes 
are installed before the tile is set. With concrete construction, 
wooden blocks may be set in the forms at the points where 
the outlets are to be located and the conduit system installed 

after the concrete is in 
place. This arrange¬ 
ment could be used for 
outlets like those shown 
in Fig. 38. Another 
method is to locate the 
conduit and outlet at 
the correct place in the 
forms and pour the concrete over them. This method is illus¬ 
trated in Fig. 39. The conduit must be properly secured, so 
that it will not be displaced during construction or when the 
wire is pulled in. Conduit which is to be buried in concrete 

* See paragraph 307. 














par. 230 ] 


RIGID CONDUIT SYSTEMS 


221 


should be galvanized and the joints made tight by the use of 
white lead or graphite. In exposed work, pipe straps (Fig. 
132) are generally used for fastening. Where a large number 


Y.oG W- X| \WW WyW<!-V- 0 V'<> 

.‘ J ‘. .<? V t 4 .\>’4 i \.<l. <1 .* .‘O’, .\*. .6«<l - Q*.‘q ■' 



* 

fflL 

-LU ■ ”■ 

1 

i 


: 

—° w 

n 

2 Iron 

1 

ooooo 

i 

! 

n tt 

2 X 2 X 

Spacers 

j 

o o o o o 

ll 

ft « ft 

^^-2 x /io 


Fig. 134. —Support for Conduit. 


of conduits are grouped together some form of hanger (Fig. 
134) is convenient. Fig. 126 shows a.concealed conduit instal¬ 
lation during construction. All wire installed in vertical runs 



Fig. 135.—Example of Exposed Conduit Installation. 


must be anchored to prevent a gradual downward creeping of 
the wires, which would put a strain on the connections. 1 he 
location of these anchors should be in accordance with Table 






































222 


METHODS OF INSTALLING WIRING [CHAP. 13 


32 (Rule 26o). Supports at these points may consist of 
approved clamps (Fig. 136) located at the ends of the conduit 
or in a junction box; or the wires may be carried on porcelain 



Fig. 136.—Method of Anchoring Vertical Risers. 

A steel junction box. with extra heavy back is placed in the run. Weight 
of cables and conduit is carried by angle iron at top and bottom which are bolted 
to the building frame work. For anchoring risers at end of run, strain insula¬ 
tors similar to these shown in Fig. 172 can be used. 


insulators supported in a junction box in such position as to 
give at least two right-angle bends in the wires. 

231. Installing Wire. Small wires may often be pushed 
through the conduit between outlet boxes, but for large wires 








1 V—_ TaDe .. ^ 



(_ 

| j f''TT~7~bisL^ Fish \\ iro 



c— 

Lv.^^YxAy.-x.y.-.-.^ I / ( f J 





Fig. 137.—Method of Pulling in Wires. 

Ends of wires are bared and twisted around a hook in the steel fish-wire. 
The ends are then taped over smoothly so they will not catch at elbows. 

or where there are a number of bends the wires must be pulled 
in. To do this a steel “ snake ” or fish-wire may be pushed 
through the conduit. This snake consists of a flat spring- 
steel wire, the size commonly used being about | in. wide and 

























































































































































par. 232 ] 


RIGID CONDUIT SYSTEMS 


223 


0.060 in. thick. The wires, if of moderate size, can be fastened 
to the end of the fish-wire and pulled in (Fig. 137). For larger 
wires a steel cable can be pulled through by means of the 
fish- wire and the wire then pulled in by means of the cable. 
W here compressed air is available, a cord may be attached to a 
piece of waste which will fit the conduit snugly, and then 
this can be blown through by means of the air. The fish-wire 
or pulling cable can then be drawn through by means of the cord. 
Before pulling in the wires, a piece of waste should be pulled 
through the conduit if there is any possibility of water having 
accumulated in the conduit, due to the condensation of mois¬ 
ture or otherwise. When pulling in the wire, grease or oil should 
never be used, as it is likely to soften and destroy the rubber 
insulation after a time. Powdered soapstone or talc can be 
safely used. Some manufacturers give their small wires a 
“ mica-finish,” which helps when pulling in the wire. When 
several wires are installed in the same conduit, they must of 
course all be pulled in together. The wires should be free from 
kinks, and should be fed in straight without twisting around 
each other. 

232 . Grounding. All conduit systems must be thoroughly 
grounded, so that a breakdown of the wire insulation will not 
charge the conduit to a dangerous potential 
(Rule 28/). Because galvanized conduit 
makes better electrical connection between 
the different parts, it is to be preferred 
to the enameled conduit where other con¬ 
ditions will permit. For the purpose of 
grounding, the conduit system is connected 
to a water pipe (on the street side of 
meter) or to a ground plate. Connec¬ 
tion to the pipes must be made by means 
of ground clamps, one type of which is 
shown in Fig. 138. Where gas pipes pass 
through outlet boxes, as in combination 
electric and gas fixtures, the gas pipe 
must be firmly connected to the outlet box (Rule 28/). 

Flexible Conduit Systems 

233 . Construction and Applications. Flexible conduit is 
made of steel strips wound spirally, forming a tube with the 



Fig. 138.—Ground 
Clamp. 













224 


METHODS OF INSTALLING WIRING [CHAP. 13 


edges of the strips interlocked in such a manner that the tube 
can be bent to a comparatively small radius. In some cases, 
one, and in other cases two strips are used in forming the 
conduit. A distinction should be made between flexible conduit 
which is made of steel and flexible tubing which is woven from 
cotton or other fabric, and is non-metallic. The flexible 
conduit is galvanized to prevent rusting. The construction 
of one make of this conduit is shown in Fig. 139. The con¬ 
duit is made in sizes from in. to 2\ ins. nominal inside 

diameter and in lengths of from 50 to 250 ft., depending upon 
the diameter. Conduit smaller than \ in. cannot be used for 
regular wiring. The smaller sizes are used only for portable 
and special work. Flexible conduit is sometimes used for ex¬ 
posed work where appearance is of no importance, but it is gen¬ 
erally used in concealed work. It is not good practice to 



Fig. 139. —Flexible Conduit. (Greenfield.) 


build it into concrete floors or walls, but it can be placed in 
grooves in a brick or tile wall and can be plastered over. Flex¬ 
ible conduit gives nearly as good mechanical protection as 
rigid conduit, although it is not absolutely nail-proof. As far 
as protection from short-circuit is concerned it is nearly as good 
as rigid conduit. The conduit is, however, not water tight and 
therefore is not as suitable as rigid conduit where exposed to 
moisture. The chief applications of flexible conduit are in 
concealed wiring for frame buildings and for extensions to a 
system in an existing building where rigid conduit could not be 
installed (Fig. 141). It is also approved by the Code for 
use in fire-proof buildings, but it is not as satisfactory as 
rigid conduit for this purpose. Since this conduit is flexible, 
no elbows are required, but the radius of bends made must 
be not less than 3| ins. (Rule 28 h). 

234. Outlets. The rules for rigid..conduit as regards outlet 
boxes apply to flexible conduit also. The same kinds of outlet 
boxes can be used for either system. Special fittings are used 




par. 235 ] 


FLEXIBLE CONDUIT SYSTEMS 


225 


to connect the flexible conduit to the boxes. Exposed ends 
of flexible conduit must also be bushed in accordance with the 
same rules as for rigid conduit.* 

235. Conduit Fittings. The lengths of flexible conduit are 
joined by clamp couplings (Fig. 140). Connections to the out¬ 
let boxes are made by 
means of fittings c or 
d. It will be seen 
that these clamp to 
the conduit and have 
standard pipe threads 
with regular locknuts. 

No b us hin g is re¬ 
quired, as the edge 
is rounded over. All 
these fittings are gal¬ 
vanized. 

236. Wire. The 

wire used in flexible 
conduit must be rub¬ 
ber insulated. It is 
the same as that used 
for rigid conduit sys¬ 
tems. Since the nom¬ 
inal inside diame¬ 
ters of flexible con¬ 
duit are the same as 
for rigid conduit, 

Table 30 can be used 
to determine the size 
of flexible conduit required for a given size of wire. 

237. Installation. The rule regarding the installation of all 
wires of an a.c. circuit in the same conduit applies to flexible 
as well as to rigid conduit. In frame buildings, the conduit 
can be run concealed in walls and floors, attaching it to the 
beams or joists by pipe straps. It is important to carefully 
fasten the bends in at least two points. If this is not done, 
it will be difficult to pull in the wire, as the bend will buckle and 
grip the wire. When the conduit is used for concealed work 
in finished buildings it is “ fished ” into the space between the 

* See paragraph 227. 



Fig. 140. —Flexible Conduit Fittings. . 

a. Conduit coupling, b. Combination coupling 
for joining to rigid conduit, c. Panel-box con¬ 
nector. d. Angle-box connector, e. Brass bush¬ 
ing for armored cables, c and d are also used for 
armored cables. 














































226 


METHODS OF INSTALLING WIRING [CHAP. 13 


walls or between the floors and ceiling. This is done by first 
pushing through a steel “snake” and then pulling in the conduit. 
For vertical runs, a chain or cord with a weight attached is 
dropped down until it meets an obstruction, and this is then 
located by the sound of the chain striking the beam. When 
installing the wire the same methods may be used as for rigid 
conduit. Vertical runs must be anchored in accordance with 



Fig. 141.—Flexible Conduit Installation. 

Showing the use of flexible conduit (Greenfield) to extend a rigid con luit 
system where an outlet must be installed after the walls have been plastered. 

• 

Table 32. The grounding of the system is accomplished in 
the same way as for rigid conduit. 

Armored Cable Systems 

238. Construction and Applications. Armored cable, fre¬ 
quently called “ BX,” the trade name of a popular brand of 
cable, consists of a flexible armor similar to flexible conduit, 
placed directly upon the wire. This wire is rubber insulated 
and covered with a braid and is in fact the same kind as that 
used in metal conduit systems. When the armored cable is 
installed in damp places or when placed in fire-proof buildings 
during construction a lead covering is required between the 
armor and the braid on the wire (Rule 27 d). Two makes of 
armored cable are shown in Fig. 142. Armored cables are made 
with single conductors from No. 14 to No. 1, twin conductors 
from No. 14 to No. 4, and three conductors from No. 14 to 
No. 6. They are furnished in coils containing from 100 to 












par. 239 ] 


ARMORED CABLE SYSTEMS 


227 


Rubber 
Braicl 
Rubber 



Fig. 142.—Armored Cable. 

b. BXL cable (leaded), c. 


a. BX cable. 
Flexsteel cable. 


250 It. In addition, armored flexible cord is also manufactured. 
Armored cable is used for the same classes of installations 
as flexible conduit. It is in fact used more commonly than the 
latter because it is cheaper and easier to install. It gives the 
same protection to the 

wire as the flexible con- —steel Armor 

duit, but has the disad¬ 
vantage that if a break¬ 
down occurs, the entire 
cable must be replaced, 
w'hereas, with a flexible 
conduit system, the 
defective wire can be 
pulled out and new wire 
inserted without disturb¬ 
ing the conduit system. 

One advantage of ar¬ 
mored cable, as com¬ 
pared with flexible con¬ 
duit, is that the cable is considerably smaller in diameter, for a 
given size of conductor, and hence can be fished in places where 
the clearance is small. 

239. Outlets. Outlet boxes similar to those already de¬ 
scribed (Fig. 122) are frequently 
used with armored cable. These 
boxes require fittings like those 
shown in Fig. 140. Another style 
of box, arranged to clamp the cable 
is shown in Fig. 143. No couplings 
are used with armored cable, as it 
must be continuous between outlet 
or junction boxes (Rule 27a). The 
arrangement of outlets is shown in 
Figs. 144 and 145. 

240. Installation. Armored cable 
cannot be installed in concrete or 
in permanently damp places unless 
the wires have a lead sheath under 
the armor (BXL style). The regular cable can, however, 
be placed in a groove in a brick wall and covered up by 
plaster of paris. When armored cable (BX) is used in the 



Fig. 143.—Outlet Box for 
Armored Cable and 
Flexible Conduit. 

The conduit is held firmly 
in place by the screws shown. 
























































































228 


METHODS OF INSTALLING WIRING [CHAP. 13 


floors or walls of frame buildings it can be run through holes 
bored in the joists or studding or it may be laid in notches 
cut in these timbers. The latter method is satisfactory, pro¬ 
vided a piece of sheet iron is nailed over the cable at these 
places to protect it from nails which might be driven into the 
cable during the process of installing the finished woodwork. 




Fig. 144. —Wall Outlet for 
Lamp Bracket. 

Showing 3-in. round outlet box 
similar to Fig. 143. 


Fig. 145.—Switch Outlet 
in Wall. 

Showing type of box adapted for 
wiring finished bulidings. 


Fig. 146 shows an armored cable installation where the joists 
were bored. » 

241. Concentric Wire. Recently experiments have been 
made with a concentric wire, which is really a form of ar¬ 
mored cable. This wire has been used extensively abroad 
but has only recently been employed in the United States. It 
is not yet* included in the Code, and is not approved for 
general use. A few installations have been made, with the con¬ 
sent of the insurance interests, for the purpose of testing the 
wire in actual service. The wire consists of a single copper 


* 1916. 




































































par. 242 ] 


METAL MOULDING 


229 


conductor, insulated with rubber, and covered by a metal 
sheath or armor (Fig. 149). This armor serves as one of the 
conductors and is carefully grounded for safety. The wire is 



Fig. 146. —Armored Cable Installations. 

Showing ceiling outlet and armored cable (BX) leading to panel box. Note 
method of supporting the outlet box. 


Metal Moulding 



intended to be used only in exposed locations for small capacity 
branch circuits. The object of this arrangement is to provide 
a cheap method of wiring 
which can be safely used 
for lighting installations 
consisting of a few lamps. 


242. Construction and 
Applications. This mould¬ 
ing consists of a sheet- 
steel trough or backing 
and a steel cover which 
is snapped on to the back¬ 
ing after the wires are in 
place. The construction 
of metal mouldingisshown 

in Fig. 150. Both backing and capping are galvanized to 
prevent rusting. Since moulding is used for small circuits, 


Fig. 147.—Wall Receptacle in Tile 
Partition. 

Showing use of armored cable (BX) with 
square box and cover for flush-plug recep¬ 
tacle. Note box connector used with cable. 





















230 


METHODS OF INSTALLING WIRING [CIIAP. 13 



Fig. 148.—Outlet on Tile Ceiling. 

Showing use of outlet plate and box 
hanger with armored cable (BX). 


Conductor 

/Rubber Insulation 


only one size (large enough to contain four No. 14 wires) is 

made. The moulding is fur¬ 
nished in 85 -ft. lengths. Metal 
moulding can be used only for 
exposed work and cannot be 
used in damp locations or for 
systems using more than 300 
volts (Rule 26/). It cannot be 
used in elevator shafts (Rule 
16 ( 7 ). Not more than four No. 
14 wires are allowed in the 
same moulding and no single 
circuit so installed may carry 
more than 1320 watts (Rule 
26/). The advantages of metal 
moulding lie in the simplicity 
of its installation and the accessibility of the wires. It is used 
chiefly for extensions 
of branch circuits in 
existing installations, 
or where there is a pos¬ 
sibility of changing the 
branch circuits fre¬ 
quently due to changes 
in the arrangement of 
the equipment in the 

room. Special outlet fittings are manufactured for use with 

metal moulding. A few 
styles of these are shown 
in Fig. 151. The vari¬ 
ous lengths of moulding 
are attached firmly to 
the outlet fittings by 
means of screws, in order 
that all parts of the 
system may be thor¬ 
oughly grounded. The 
figure shows some of 
the fittings used for 
making taps, turning 
corners, etc. The wire used may be either rubber insulated, 



v ~ Braid 


^—Metal Sheath 


Fig. 149.—Concentric Wire. 

(General Electric Co.) 



Fig. 150.—Metal Moulding. 

a. “ National.” b. “ Pagrip.” 



























































par. 243 ] 


METAL MOULDING 


231 


single conductor, single braided or rubber insulated twin 
wire. 



Fig. 151.—Metal Moulding Fittings. 

a. Elbow, b. Tee. c. Receptacle. The moulding is attached to the fit¬ 
tings by the screws shown in the illustrations. 


243. Installation. Metal moulding may be cut with a fine 
tooth hack saw or may be marked with a three-cornered file and 
broken. If the capping is in 
place, the moulding may, with 
care, be bent to any radius not 
less than 4 in. Special tools are 
made by the manufacturers 
for cutting the moulding and 
punching the screw holes. These 
tools save a great deal of time. 

The backing is secured to the 
ceiling or wall by wood screws 
or toggle bolts; the wire is laid 
in the backing and the capping 
snapped into place. When the 
moulding passes through a floor 
it must be protected by an 
iron pipe extending from the 
ceiling line to a point at least 3 
in. above the floor (Rule 296). 

Where the moulding passes 
through a partition, a pipe is not 
required, provided the surround¬ 
ings are dry and the moulding 
has no joint inside the partition. 

Fig. 152 shows an installation 
of metal moulding. All parts 
of the moulding system must be thoroughly grounded as 
described for metal conduits. 



Fig. 


152.—Metal Moulding 
Installation. 


A single ceiling outlet (c) is re¬ 
placed by several pendants over 
counter. (Nat’l Metal Molding 
Co.) 





































































































232 


METHODS OF INSTALLING WIRING [CHAP. 13 


Wood Moulding 

244. Construction and Applications. Wood moulding con¬ 
sists of a backing with grooves to contain the wires and a cap¬ 
ping which is nailed to 

- lYo- - 


the backing after the 
wires are in place. The 
Code recommends the 
use of hard wood mould¬ 
ing, but generally soft 
wood is used. The cost 
of hard wood moulding 
is nearly twice that of 
soft wood. Fig. 153 
shows the construction. 
Moulding is made for 
two wires and for three 
wires, in sizes from No. 
14 to 400,000 cir. mils. 
The size holding wires 
up to No. 12 is most 
commonly used. The 
wires must be separated by a tongue at least \ in. thick and 
the moulding must be painted or shellacked inside and outside 



Fig. 153. —Wood Moulding. 

a. Two-wire. b. Three-wire. Sizes shown 
are for No. 14 or 12 wire and are those most 
commonly used, 



a 

Fig. 154. —Fittings for Wood Moulding. 

a. Tap. b. Rosette. 


to exclude moisture. Wooden moulding can only be used for 
exposed work. It is not allowed in damp places, in elevator 
shafts (Rule 16^), or for systems operating at more than 300 




































































































par. 245 ] 


KNOB AND TUBE SYSTEM 


233 


volts (Rule 26 1). Wood moulding is often used for extensions 
of branch circuits in existing installations. In wiring old build¬ 
ings, wood moulding is sometimes used on the ceilings, with 
flexible tubing or armored cable where the wires are concealed 
in the walls. Wood moulding is cheaper than metal moulding 


but in general it is not as satisfactory, 
the local rules of some of the large 
cities. Because of its large size, wood 
moulding is usually more conspicuous 
than metal moulding, unless it is com¬ 
bined with the trim of the room. By 
the use of ■ special capping to imitate 
picture moulding or by arranging it in 
panels on the ceiling it can be made 
less conspicuous. For outlets, por¬ 
celain receptacles or rosettes are used. 
Taps or cross-overs must be made by 
means of special fittings (Rule 26 k), 
(Fig. 154). Rubber-insulated wire 
must be used in moulding (Rule 26k). 
Wood moulding cannot be run through 
floors or partitions. In such cases, 
the wires must be run in iron pipe 
or porcelain tubes (Rule 16d). Where 
the wires enter the moulding, if at the 
floor line, a wooden or steel box 
(“ kick block ”) (Fig. 155), must be 
to protect them from injury. 


Its use is forbidden by 



Fig. 155. —“Kick Block.” 

For protecting wires in¬ 
stalled in wood moulding 
when passing through floor. 


placed around the wires 


Knob and Tube System 

245. Description and Application. The cheapest form of 
concealed wiring is the knob and tube system. The wires are 
run beneath the floors or in the partitions and are supported on 
porcelain insulators or knobs where the wires are run parallel 
to floor joists or studding, and pass through porcelain tubes 
where crossing beams, partitions, etc. Figs. 156 and 157 illus¬ 
trate this type of wiring. The system is used chiefly in frame 
buildings (dwellings, etc.), where a cheap piece of work is 
desired. The use of the knob and tube system is prohibited 
by the local rules of many large cities. It cannot be used for 



























234 


METHODS OF INSTALLING WIRING [CHAP. 13 


fire-proof buildings or for damp places. With this system, the 
wires are not protected from mechanical injury, and there is 
alwaj^s the possibility that they may be damaged by workmen 
during construction or by rats after the house is in use. The 
wires may sag against beams, lath, etc., or they may be cov¬ 
ered by shavings or other inflammable material during construc¬ 
tion so that an overheating of the wires or a short-circuit might 
start a fire. The knobs may be either solid or split (Figs. 158a 
and b ), and must keep the wires at least 1 in. above the surface 
wired over. Knobs are generally held by wire nails using a 
leather washer or nail head to prevent breaking the knob, The 



Fig. 156. —Knob'and Tube Wiring under Floor. 


tubes have a head at one end to prevent their being displaced 
(Fig. 158c). The wire used must be rubber-insulated, with 
single braid up to No. 6 gauge and double braid for No. 6 and 
larger sizes. No outlet boxes are required, although steel 
outlet boxes or plates are recommended (Rule 26m) (see Fig. 
157). At all outlets, the wire must be protected by flexible 
tubing (Fig. 159), which extends from the last knob to at least 
1 in. beyond the surface (see Fig. 34). A similar arrangement is 
required for switch outlets. If flush switches or receptacles are 
used they must be enclosed in steel boxes (Rule 24 d). The 
wires must be kept as far apart as possible, separated, at least 5 
in. and run on different studding wherever possible (Rule 26r). 
The wires must be supported at least every 4.5 ft. and at shorter 
intervals if they are liable to be disturbed. At outlets and 
















































par. 246 ] 


OPEN WORK 


235 


panel boxes, where the 5-in. separation cannot be maintained, 
the wires must be covered by flexible tubing (Fig. 157). Where 
it is impossible to use insulators, the 
wires may be fished if each is sepa¬ 
rately incased in flexible tubing (Rule 
26s). For wires carrying more than 
300 volts or for damp places, flexible 
conduit or armored cable must be 
used. The flexible tubing used is 
sometimes called “ circular loom.” 

The construction differs slightly with 
various manufacturers, but essen¬ 
tially it consists of a seamless tube 
built up of braided coverings com¬ 
bined with a closely wound cord or 
flat paper strip and thoroughly im¬ 
pregnated with a waterproof com¬ 
pound (Fig. 159). Flexible tubing 
will resist considerable abrasion, 
but is, of course, not nail-proof and 
is easily crushed. 


Open Work 

246. Description and Applications. 

In open work, the wires are run 
exposed, supported on porcelain 
knobs or cleats. This system is 
used for small branch circuits, where 
appearance is of no importance, as 
in cellars, and is also used frequently 
for heavier circuits, for factories. In such cases, the feeders and 
mains are run near the upper part of the room or on the ceiling 
where they are not likely to be damaged. The branch cir¬ 
cuits, which are subject to possible injury and displacement, are 
run in conduit. This arrangement is much cheaper than where 
conduit is used throughout, and in many cases is just as satis¬ 
factory. The advantages of open wiring are that it is cheap 
and accessible and can be easily changed as required. On the 
other hand, the wires are not protected, and are liable to 
mechanical injury. 



Fig. 157.—Knob and Tube 
Wiring in Partition. 

Extra porcelain tubes must 
be placed over wires at floor 
line to protect them from 
plaster dropping down during 
construction. 

















































236 


METHODS OF INSTALLING WIRING [CHAP. 13 


247. Cleats and Insulators. For wires smaller than No. 8, 

split knobs (Fig. 158) or cleats must be used (Rule 165). For 
larger wires solid knobs may be used, but the better arrange¬ 
ment is to use suitable cleats. For small branch circuits, up 
to about No. 10, two- or three-wire cleats are generally used 



Fig. 158.—Porcelain Knobs and Tube. 

a. Split knobs, b. Solid knobs, c. Tube. 


(Fig. 160). These cleats are fastened by screws and grip the 
wires firmly when properly installed. For large wires, the best 
support is the single-wire cleat, provided it is heavy enough to 
stand the strains which occur during installation of the wire. 
Where only one or two circuits are to be run together the type 

of cleat shown in Fig. 
161 is satisfactory. 
Table 33 gives dimen¬ 
sions of these cleats. 
Where a large number 
of heavy wires must 
be run close together, 
as in wire tunnels, back 
of switchboards and in 
similar places, some form of insulator rack is desirable. The 
style shown in Fig. 162 is useful for such work. Racks are often 
built from strap or angle iron to suit the requirements of the 
particular location. A good example of this type of construc¬ 
tion is shown in Fig. 163. 

248. Protection of Wires. Wherever wires pass through 
floors or walls, they must be protected by porcelain tubes 


























































































































par. 248] 


OPEN WORK 


237 


(Rule 16 d) (fig. 164). For wires entering a building from an 
overhead line, the arrangement "shown in Fig. 165 is satisfactory. 
Wherever the wires may come in contact with pipes or other 
conducting ma¬ 
terial, porcelain 
tubes must be 
placed over the 
wires to keep them 
out of contact 
(Pig. 166). When 
crossing other 
wires, porcelain 
tubes or an equiv¬ 
alent device must 
be used to sepa¬ 
rate the wires (Fig. 

167). The tube 
should be put on 
the wire which is 

nearest the surface wired over, to keep the other wire which 
is not in a tube clear of this surface. Where wires are ex- 



Fig. 160.—Porcelain Cleats. 

a. Two-wire, b, Three-wire. 



Fig. 161. — Single-wire Cleats. 

posed to mechanical injury they must be. suitably protected 
(Rule 26e). When crossing floor timbers they may be run 







































































































































































































































































































238 


METHODS OF INSTALLING WIRING [CHAP. 13 


on the under side of a wooden strip 3 in. wide and not less 
than f in. thick, or guard strips dn either side of the wires may 

be used (see Fig. 168). On side 
walls, the protection must extend 
not less than 7 ft. from the 
floor. The wires may be enclosed 
in wooden boxing, closed at the 
top, or iron conduit may be 
used. Iron conduit is prefer¬ 
able, except in damp places (Fig. 
169). 

249. Wire. For open wiring, 
rubber-covered wire (with single 
braid up to No. 6 gauge and 
double braid for larger sizes) is 
the best and is frequently required by local rules. The Code 



Fig. 162. —Cable Rack and 
Insulators. 

(General Electric Co.) 



Fig. 163. —Large Group of Wires on Racks. 

(Factory Mutual Ins. Co.’s.) 


allows the use of slow-burning insulation* under certain re- 

* See paragraph 25S. 




































par. 249 ] 


OPEN WORK 


239 


strictions. For dry locations, slow-burning insulation is satis- 




^ Iron cpnduir ^ a ^ ■=* 



Fig. 164.—Protection of Wires Passing through Walls. 

a. Thin wall. b. Thick wall where a single tube would be too short. The 
hole is bushed with a piece of iron conduit and the tubes entered at each end. 

factory, and is commonly used in factories for the feeders or 
mains when they are run exposed, as these circuits can usually be 


located in places where they 
are not likely to be dis¬ 
turbed. The objection to 
the use of slow-burning wire 
is that the porcelain knobs 
or cleats which support the 
wire must be depended 
upon to maintain proper 
insulation. Where there is 
much moisture present there 
is difficulty in doing this. 
In damp places, therefore, 
rubber-insulated wire is re¬ 
quired (Rule 26 i). 1 he 

advantages of slow-burning 
insulation are that it is 
cheaper than rubber and 
is not as inflammable. It 
has the further advantage 



Fig. 165.—Protection of Wires 

Entering Building. 

Wires enter through porcelain bushings. 
Note drip loop and bushing slanting in 
such a way as to exclude water. (Factory 
Mutual Ins. Co’s.) 


that the smooth surface will not 





































































240 


METHODS OF INSTALLING WIRING [CHAP. 13 


easily collect dust. Since the use of slow-burning wire is 
not always approved, it is well to consult the local in¬ 
spection authorities before using it in an installation. 



Fig. 166.—Protection of Wire Crossing a Pipe. 

Tube is taped to wire to prevent it sliding along wire. When tube is not 
located next a cleat, each end of tube should be taped. Wire should be installed 
above the pipe where possible to prevent condensation from pipe accumulating 
on the wire. 


250. Installation. For open work, the wires must be so 
supported on insulators as to give the following minimum 
spacings (Rule 26 h). 


\ 

Distance from Surface 

Distance 


Wired Over. 

between 

Voltage. 







\V ires. 


Dry, In. 

Damp, In. 

Ins. 

0 to 300 volts. 

1 

2 

1 

Ol 

"2 

301 to 550 volts. 

1 

1 

4 


Supports must be located not more than 4.5 ft. apart, or closer 
if necessary to keep the wires out of contact with surrounding 
surfaces. Wires which are not likely to be disturbed may be 
spaced 6 in. apart and run from beam to beam, even when 
the supports are more than 4.5 ft. apart. Smaller wires must 
follow the surface of the ceiling and “ break around ” the 
beams, unless they are protected by guard strips (Fig. 170). 
Small wires may be anchored or “ dead ended ” on the cleats 
(Fig. 171). Large wires, especially long runs, must be anchored 













































PAR. 251] 


COMPARISON OF SYSTEMS 


241 


by strain insulators (Fig. 172). When installing supports 
for open wiring, particularly for large wires, care is necessary 
with the fastenings. On wooden surfaces or on beams, girders 
or trusses it is easy to obtain suitable supports for the insulator 
racks. When the rack must be fastened to a brick wall, an 



Fig. 167.—Tap on Open Wiring. 

One joint left untaped. Note position of tube where wires cross. 

expansion bolt or a lead plug should be used. W ooden plugs 
should never be employed, as the wood is sure to dry out in 
time and the screw become loosened. Fig. 173 illustrates open 
wiring construction. 


Running Board SWl'x %" 



Fig. 16S.—Protecting Wires from Injury. 

a. Using running board, b. Using guard strips. 


251. Comparison of Wiring Systems. As far as mechanical 
protection is concerned, the rigid conduit system is the best, 
with flexible conduit and armored cable nearly as good. These 
systems are therefore the best lor use where the wiies may be 
damaged if not suitably protected. The other systems are 
for use in locations where the wires will not be disturbed. As 
far as the fire risk is concerned, the rigid, flexible conduit and the 








































1 


242 


METHODS OF INSTALLING WIRING [CHAP. 13 


armored cable systems are equally good for moderate sizes 
of circuits. Rigid conduit will stand a heavier arc than will 
the flexible conduit or the armored cable, and consequently 
it is better for heavy circuits, where the arc due to a short-cir- 



Fig. 169.— Protecting Wires 
on Side Walls. 

a. Wooden boxing. b. Iron 
conduit protection. Flexible 
tubing must be used the entire 
distance between the cleats at 
either end unless rubber-covered 
wire is used, when it can be 
omitted. Bushings at ends of 
conduit must be used in either 
case. 



Fig. 170.—Method of Breaking 
around Beams. 



Fig. 171.—Method of Anchoring 
Small Wires. 

Sometimes two cleats are placed side 
by side and the ends of the wires carried 
over the two and twisted as shown. 


cuit might be very severe. As far as ease of installation is con¬ 
cerned, the flexible conduit or armored cable system stands 
ahead of the rigid conduit. This applies to concealed work, in¬ 
stalled in either new or finished buildings. For new buildings, 
of course, the knob and tube system is simpler to install than 




























































































PAR. 251 ] 


COMPARISON OF SYSTEMS 


243 


either. For exposed work, open wiring is the simplest to 
install. A decision as to the kind of wiring to be used is affected 
principally by the matter of cost. While this item is of first 
importance, the system which costs the least to install is not 



always the best or the cheapest in the end. The character of 
the building and the service has to be considered. If a system 
in a factory or an office building is so cheaply installed and of 
such poor construction that it is unreliable, there will be con- 



Fig. 173.—Example of Open Wiring. 

(Factory Mutual Ins. Co’s.) 

tinual interruptions to the service. In a factory the loss caused 
by stopping work due to those interruptions might easily be 
much greater than the difference in cost between a cheap system 
and one which is entirely reliable. In the case of an office 
























244 


METHODS OF INSTALLING WIRING [CHAP. 13 


building unreliable service would affect the rental value of the 
property. In either case, the cost of repairs and maintenance 
would be greater than for a better system. The relative cost 
of the various systems installed in new buildings is indicated 
below; 


Relative Cost of Wiring Systems 

Per Cent. 


Rigid conduit, concealed. 100 

Rigid conduit, exposed. 125 

Armored cable, new work. 65 

Knob and tube wiring. 40 

Open wiring. 50 


This cost would vary with the type of building and would be 
different for work installed in old buildings. Changes in the 
cost of labor and material would also affect these figures. 

252. Wiring of Finished Buildings. In many cases, the 
wiring must be installed in finished buildings where no pro¬ 
vision was made for electric wires at the time the buildings were 
constructed. Such buildings include residences and very old 
factory or office buildings. Where the wiring must be con¬ 
cealed, as would usually be the case in residences and office 
buildings, either the knob and tube or the metal enclosed sys¬ 
tem (rigid or flexible conduit or armored cable) may be used. 
In a number of large cities, knob and tube wiring is not allowed. 
For metal-enclosed systems, rigid conduit is the best, but this 
usually requires much more cutting and removal of floors. 
Armored cable or flexible conduit can be fished and hence is 
easier to install. If either of these systems is used for damp 
places, however, the wire must be covered by a lead sheath. 
Armored cable or flexible conduit can, however, be laid in a 
groove in a brick wall and plastered over without requiring a 
lead sheath, provided the wall is not continually damp (Rule 
27 d). Frequently rigid and flexible conduit may be used in 
the same installation, thereby reducing the cost and obtaining a 
result nearly as good as an entire rigid conduit system. Such an 
arrangement could be used where a portion of the system is 
exposed to dampness. Where knob and tube wiring is allowed, 
this makes the cheapest system. It would be usual to remove 
floor boards, where required, and install the horizontal runs on 







PAR. 253 ] WIRING FOR SPECIAL CONDITIONS 


245 


knobs. On the vertical runs, flexible tube would be used in 
wood partitions, and metal conduit or armored cable for brick 
or stone walls. For exposed work, the installation would be no 
different from those already described in previous paragraphs. 
Rigid conduit or armored cable is rather conspicuous, but 
makes the safest job. Metal moulding is fairly safe and can 
be made inconspicuous. Wood moulding is not very safe. It 
can be made in conspicuous, but its use is prohibited in some 
cities. Cleat or open wiring is unsightly and is exposed to dam¬ 
age, but it is, of course, cheap. 

253. Wiring for Severe Conditions. For extremely wet 
places, such as canning plants, slaughter houses and breweries, 
special precautions must be taken 
with the wiring. Both open wiring 
and rigid conduit systems have 
been used. With open work, the 
wires are provided with a double 
thickness of rubber insulation and 
are run about 6 in. apart, supported 
on porcelain insulators or knobs 
(Fig. 174). For outlets, keyless 
sockets, made of moulded insula¬ 
tion, are preferable. The joints must 
be carefully made, the rubber com¬ 
pound being warmed and pressed 
firmly around the w T ire. The joints 
should then be covered with fric¬ 
tion tape and heavily coated with insulating paint. Fuses and 
switches should, where possible, be placed outside the rooms and 
enclosed in substantial cabinets. The open system is objection¬ 
able because it is liable to be disturbed by operatives or workmen, 
and with the expensive wire required and the special supports 
it costs as much or more than a conduit system. When a rigid 
conduit system is used, ordinary enameled conduit can be em¬ 
ployed if the conditions are not too severe, particularly if it is 
thoroughly painted after installation. Galvanized conduit is, 
however, more satisfactory and must be used where the atmos¬ 
phere contains corrosive vapors such as exist in tank rooms, 
glue houses and fertilizer rooms of packing plants. Conduits 
for such places should be “ hot galvanized.’’ Cast-iron outlet 
boxes should be used and the covers should be provided with 



Fig. 174.—Wiring for Wet 
Places. 




















246 


METHODS OF INSTALLING WIRING [CHAP. 13 


rubber gaskets. The conduit should screw into the boxes and 
should be made tight by means of white lead. The conduit 
should be repainted carefully at all joints and fittings and the 
entire run painted at intervals. It is also best to keep the 



Fig. 175.—Method of Preventing Condensation. 

a. Draining a horizontal run. b. Preventing circulation of air in conduit, 
c. Draining a vertical run. 

switches and cutouts outside the rooms where the severest con¬ 
ditions exist. Where different parts of the conduit run are sub¬ 
jected to different temperatures, care must be taken to prevent 
water accumulating from condensation in the conduit. If cold 

air can circulate in the conduit system and 
come in contact with warmer air, the mois¬ 
ture in the latter will be condensed and 
accumulate in the conduit. The conden¬ 
sation can be stopped by preventing a circu¬ 
lation of air by plugging the conduit (Fig. 
1756). Where steam is present or where 
the temperature of the room changes, the 
conduit can be drained into either an outlet 
box or cabinet, or by providing a fitting with 
an opening at the lowest point on the run 
Iig. 176. Vapor- (Figs. 175a and c). Flexible conduit with 
tight Outlet. lead-covered wire has been used in some 
cases, but this will not withstand as severe 
conditions as the other systems. In all cases, it is important 
that the workmanship be of the highest grade. Even material 
of the best quality will not give satisfaction if carelessly 
installed. For storage-battery rooms, where sulphuric acid 





















































































PAR. 253 ] WIRING FOR SPECIAL CONDITIONS 


247 


fumes are plentiful, single conductor, lead-covered rubber 
insulated wire has been used successfully. Taps are made 
by removing the lead covering, connecting and insulating 
with rubber, then covering with friction tape, which is car¬ 
ried over the lead on each side. The tape is then painted 
over with insulating paint. Joints of this kind must, how¬ 
ever, be carefully made in order to be satisfactory. In 
such cases the switches would be located preferably outside 
the room near the entrance. In rooms where inflammable 
gases exist, the lamps must be encased in a vapor-tight globe, 
such as is shown in Fig. 176 (Rule 31a). Where wiring is sub¬ 
ject to high temperatures (above 120 degrees Fahr.), rubber 
insulation rapidly deteriorates. For such places slow-burning 
or asbestos insulation is required. Fixtures for large gas-filled 
lamps used indoors should not be wired with rubber insulated 
wire* (Rule 3 5d). 


* See paragraphs 33 and 86. 








CHAPTER 14 

WIRES AND CABLES 

254. Materials Used for Conductors. The metais which 
have been used most commonly for conductors are copper, 
aluminum and iron. German silver, lead and various alloys 
are used in electrical apparatus, such as heaters, fuses, etc., but 
they are never used for transmitting electricity for lighting or 
power service because of their high resistance. Of the three 
metals mentioned above, copper is most commonly used for 
power transmission and interior wiring because of its low 
resistance and relatively small cost. Iron is sometimes used out¬ 
doors for very long spans where copper would not be strong 
enough mechanically. Iron wire has a resistance from 6 to 8.25 
times that of annealed copper, depending upon the quality of the 
material. Ordinary steel rails have a resistance of 11 to 13 times 
that of copper. Special steel rails which are used for third rails 
may have a resistance as low as 8 times that of copper. Iron 
or steel wire is used extensively for telephone and telegraph 
circuits where high resistance is not a serious disadvantage. It 
is never used for interior wiring, although steel rails or structural 
shapes are sometimes used for conducting current to cranes. 
Copper-clad or bi-metallic wire is composed of an iron or steel 
wire covered with a heavy coating of copper. The resistance 
of the wire depends upon the relative amounts of copper and 
iron and is expressed by the manufacturers as the per cent con¬ 
ductivity compared with a copper wire of the same size. Wires 
having respectively 30 per cent, 40 per cent and 47 per cent of 
the conductivity of copper are standard. This wire is chiefly 
used for long telephone and telegraph lines and to some extent 
for power transmission. It is not used for interior wiring. Alu¬ 
minum is used rather extensively for long-distance power trans¬ 
mission systems where bare conductors are employed. An 
aluminum wire is, however, considerably larger in diameter 
than a copper wire having an equal resistance. Thus, if we 

248 


par. 254 ] 


COPPER WIRE 


249 


require a copper conductor of 1,000,000 cir. mils area, an 
aluminum conductor of the same resistance per foot would 
have to be 1,590,000 cir. mils in area. The diameter of the 
aluminum cable would be about lj times as great as the cop¬ 
per wire. For equivalent sizes, the weight of aluminum wire 
is less than half that of copper. This is sometimes an advan¬ 
tage where heavy lines are used. The cost of bare aluminum 
wire for the same resistance is about the same as for copper. 
Insulated aluminum wires would, therefore, cost more and would 
require larger conduits than copper wires. It is also very diffi¬ 
cult to solder aluminum. These disadvantages prevent'the 
general use of aluminum for interior wiring. The following 
discussion of insulated wires will, therefore, be confined to 
copper conductors. 1 he copper used for electrical purposes 
must be very pure. Even slight amounts of different metals 
or other impurities greatly increase the resistance. Nearly all 
the copper used for electrical conductors is refined electrically. 
In one of the methods used, the crude copper is cast into the 
form of heavy plates which are hung in a tank containing a solu¬ 
tion of copper sulphate (blue vitriol) and other substances. 
These plates are connected to the positive terminal of an 
electric circuit, the negative terminal being connected to thin 
plates of pure copper which are hung close to the crude cop¬ 
per plates. The copper is deposited on the negative plates 
by the action of the current and the impurities in the crude 
copper drop to the bottom of the tank. The pure copper 
plates are then removed from the tanks, melted in a furnace 
and cast' into bars about 4 in. square. These bars are rolled, 
while hot, until the cross-section is reduced to a rod about 1 in. 
in diameter. After cooling, the rod is cleaned from scale and 
is then drawn through a tapering hole in a steel plate. There is 
a limit to the reduction which can be made by passing through 
the steel plate or “ die,” so that for fine wires the drawing process 
must be repeated a large number of times. The drawing tends 
to harden the wire and to increase its tensile strength and 
stiffness. After a certain amount of drawing has taken place 
the wire becomes too hard and must be “ annealed ” by heating 
in a furnace. This makes the wire “soft” again. Hard-drawn 
and medium hard-drawn copper wires are produced by proper 
wire drawing and are not annealed after the drawing process 
is completed. These wires are stronger and more elastic than 


250 


WIRES AND CABLES 


[chap. 14 


annealed wire and are, therefore, used chiefly for outdoor pur¬ 
poses where mechanical strength is necessary. The principal 
applications are for trolley wire, transmission lines, telephone 
and telegraph lines. The resistance is about 2.7 per cent 
greater than for soft copper. Annealed or soft copper wire is 
produced by carefully annealing after the drawing process is 
completed. The wire can be easily bent, but is only about 60 
per cent as strong as hard-drawn wire. It is used for the wind¬ 
ings of electrical machinery and for insulated wires used for 
interior wiring. 

255. Wire Gauges. Copper wires in the smaller sizes (up 
to about rg- in. diameter), are measured by the Brown & Sharpe 
(B. & S.), or American Wire Gauge (A. W. G.). The sizes run 
from No. 40, which is 0.0031 in. in diameter, to No. 0000, 
which is 0.460 in. in diameter. Larger sizes are designated 
in circular mils, the usual sizes running from 250,000 cir. 
mils, which is slightly larger than No. 0000, to 2,000,000 cir. 
mils which is 1.631 in. in diameter. The sizes usually made 
advance by 50,000 cir. mils up to 1,000,000 and by 100,000 
cir. mils from this size to 2,000,000. In the B. & S. gauge, the 
wires double in cross-section for every three sizes. Thus, a 
No. 7 wire is exactly twice the area of a No. 10 wire. A No. 
4 wire is twice the area of a No. 7 wire, etc. A No. 4 wire 
will not, however, carry twice the current that is safe for a 
No. 7 wire (see Table 36). The diameters of round wires are 
sometimes expressed in mils. A mil is one thousandth of an 
inch. Thus a No. 10 wire is 0.102 in. or 102 mils in diameter. 
The areas of wires are frequently expressed in circular mils. A 
circular mil is the area of a circle one mil or 0.001 in. in 
diameter. The area of any round wire expressed in circular 
mils is found by squaring the diameter of the wire, measured 
in mils. Thus, a No. 10 wire has a diameter of 102 mils, and, 
therefore, the area is: 102X102 = 10,404 cir. mils, or 0.00816 
sq.in. 

Rectangular areas are sometimes measured in square mils. 
This is found by multiplying together the dimensions measured 
in mils. Thus a bar 3 in. by \ in. measures 3000 mils by 250 
mils. The area is, therefore, 3000X250 = 750,000 square mils, 
or 0.75 sq.in. Sometimes it is desired to find the number of 
circular mils in a rectangular section. To change from one 
system to the other, we have, 


par. 255 ] 


WIRE GAUGES 


251 


circular mils = square mils X 1.273; 
square mils = circular milsXO.7854; 
circular mils = square inches X 1,273,000; 
square inches = circular mils XO.0000007854; 
square inches =square milsXO.000001; 
square mils = square inches X 1,000,000. 


For the copper bar, therefore, the area is 750,000X1.273 = 
955,000 cir. mils. The area of a wire is of importance, since the 
resistance decreases exactly as the area increases. Thus a 
1,000,000 cir. mil cable is one-half the resistance per 1000 ft. 
of a 500,000 cir. mil cable, and a 2,000,000 cir. mil cable is one- 
quarter the resistance of the 500,000 cir. mil cable. The B. & S. 
gauge sizes and the corresponding area in circular mils are 
given in Table 36. In finding the resistance of a conductor 
the resistance of a wire one mil in diameter and one foot long is 
used as a standard. This is called a mil-foot. The value of the 
resistance per mil-foot for commercial copper wire at 75° Fahr. 
is 10.75 ohms. The resistance of a wire can be calculated from 
the formula: 


Resistance = 


10.75Xlength in feet 
cir. mils. 


d) 


Example 1. Find the resistance of a wire having a cross-section of 
16,500 c.m. and 1500 ft. long. 

10.75X1500 

Resistance =- 

16,500 


= 0.978 ohm. 


The size of wire for a given resistance is found by the formula: 


Cir. mils = 


10.75Xlength in feet 
Resistance in ohms 


( 2 ) 


Example 2. Find the size of wire to have a resistance of 0.2 ohm 
when the length is 2000 ft. 

10.75 X20,000 

C.m. =-- 

0.2 

= 107,500 c.m 

Example 3. Find the resistance of a rectangular bar 3 ins. by J in. 
and 20 ft. long. From previous calculations, the size was found to be 
955,000 c.m. Hence the resistance is: 

10.75X20 

Resistance =- 

955,000 

= 0.000225 ohm. 







252 


WIRES AND CABLES 


[chap. 14 


266. Solid and Stranded Conductors. Solid conductors are 
composed of a single wire. Sometimes this is called a solid wire 
or simply a wire. Stranded conductors are composed of a 
number of wires twisted together. This is generally called a 
cable. A very small stranded conductor is usually known as a 
stranded wire. Twin wire or duplex wire consists of two sep¬ 
arately insulated wires, laid parallel and having an outside braid 
which encloses both wires. For interior wiring, where the con¬ 
ductor is installed in conduit, it is usual to employ stranded wires 
or cables in sizes No. 8 and larger, as it is difficult to pull large 
solid wires into conduit. Where the wires are run open, there 
is not this objection to the use of solid wires, but in this 
case it is advisable to use stranded wires in sizes above 



Fig. 177.—Cables. 


a. Rope stranding, b. Concentric stranding. 

No. 1 because of the difficulty of' taking the kinks out of 
a heavier solid wire so as to give a good appearance. Cables 
are almost always used for sizes larger than No. 0000. 
Cables are slightly more expensive than solid wires. There are 
two kinds of cables in use. For rope-laid cables (Fig. 177a), the 
wires are first twisted together in groups of sevens, and these 
sevens are then combined to make up the cable. This arrange¬ 
ment makes a more flexible cable than the other type, but the 
diameter is greater for a given cross-section of copper. It is 
used principally for making very large, extra-flexible cables. 
The concentric-laid cable uses a number of wires all of the same 
size, laid up in layers around one central wire (Fig. 1776). 
This is the common type of cable used for electrical conductors. 
Cables can be made up of different numbers of wires, the smaller 
the wires, the greater the flexibility of the cable. The cost, 
however, is greater for a large number of fine wires. The stand- 




par. 257 ] 


RUBBER INSULATION 


253 


arcl strandings adopted by the manufacturers are chosen so as to 
give sufficient flexibility for all wiring purposes, except portable 
or 'machine cables. In Table 34 is given the strandings gen¬ 
erally used. With the help of this table, the size of a stranded 
wire can be easily determined by measuring the diameter of 
one wire and counting the number of wires or strands. The 
values given in heavy type are the ones most commonly used. 
It will be seen from this table that the size of wires used for 
ordinary cables varies from about No. 16 for small-size cables to 
No. 8 for larger sizes. The number of strands is never less than 
seven. For extra-flexible wires, such as lamp cord, smaller indi¬ 
vidual wires are used. Thus, ordinary No. 18 lamp cord is 
composed of sixteen No. 30 gauge wires. Extra-flexible cables 
such as are used for dynamo leads are composed of a large num¬ 
ber of fine wires and are sometimes rope laid. 

Insulated Conductors 

f 

257. Rubber Insulation. The rubber insulation used on 
wires and cables is composed of from 20 to 40 per cent of India 
rubber, the remainder being usually mineral matter of various 
kinds, such as talc, zinc oxide, red lead, etc., with a small amount 
of sulphur. Pure rubber cannot be used for wire insulation 
because it will not stand high temperatures and is too soft to 
withstand the rough usage to which wire is subjected. The 
rubber, after being washed, is “ compounded ” or mixed with 
the mineral matter by working through heated rolls, until it is 
thoroughly plastic. The compound is then rolled out into thin 
sheets and cut into strips suitable for applying to the wire. 
Copper wire which is to be covered with rubber insulation is 
always thoroughly tinned to protect the copper from the cor¬ 
roding action of the sulphur in the compound. For small wires 
the compound is applied by pressing it through a die, at the 
centre of which the wire is located. Large wires usually have 
the insulation applied in the form of a strip which is carried 
along parallel to the wire and is folded over and compressed 
tightly around it by means of grooved rollers. After the in¬ 
sulation is applied, it is vulcanized by heating to the proper 
temperature. This is usually done by placing the coils of wire 
in large, closed drums and admitting live steam at a pressure 
of about 25 pounds. This process causes the sulphur to com- 


254 


WIRES AND CABLES 


[chap. 14 


bine chemically with the rubber, with the result that it is no 
longer plastic but firm, elastic, strong, and less affected by heat 
or cold or the action of the air. After the insulation has been 
vulcanized, the wire is covered by one or two cotton braids, 
which are thoroughly filled with a weatherproof compound. 





Braids 


Outside braid, finished smooth 
with weatherproof com 


Flameproof braid 



Rubber insulation 



Fig,. ITS.—Insulated Wires and Cables. 

a. Rubber-covered cable, b. Triple braid weatherproof cable, c. Slow- 
burning cable. Slow-burning weatherproof cable is similar except that the 
inner braid is weatherproof, d. Fixture wire. e. Duplex wire. 

Sometimes, on large wires, a tape is used instead of the inside 
braid (Fig. 178). The character of the rubber insulation de¬ 
pends to a considerable extent upon the quality of rubber used 
and also upon the percentage of rubber in the compound. For 
many years, the best rubber has been obtained from Brazil, the 
brand called “ Para ” being the most satisfactory. African 
rubbers are in general inferior in quality. Insulation manu- 



































































WEATHERPROOF INSULATION 


255 


par. 258] 

< 

factured to meet the Code rules requires about 20 per cent of 
rubber, but it does not need to be of the highest grade. A com¬ 
pound containing 30 per cent Para rubber is made by a number 
of manufactuiers. This insulation is much superior to the Code 
insulation and is used for important work and for high-voltage 
installations. A grade of insulation intermediate between Code 
insulation and 30 per cent Para insulation is also manufactured. 
This is much better than the Code insulation and is used in many 
high-class installations where freedom from breakdowns is impor¬ 
tant. No rubber insulation poorer than that required by the 
Code should ever be used for electric light and power wiring. 
Tests of rubber insulation are made by the manufacturers and 
sometimes by the purchaser. The principal tests are a high-volt¬ 
age test followed by a test of the insulation resistance. Both of 
these are made while the wire is immersed in a tank of water. 
Samples of the insulation are also stretched under specified con¬ 
ditions. The dimensions of the standard Code wire for voltages 
up to 600 volts are given in Table 35. Fixture wire is insulated 
with a wall of rubber thinner than that on the regular wire. 
Nothing smaller than No. 18 wire is allowed, and No. 16 is 
preferable where it is possible to use it. As a rule, solid wire is 
used, but stranded fixture wire can be obtained. The wire is 
covered with a single braid. Fixture wire (Fig. 178d) is con¬ 
siderably smaller than the regular No. 14 wire, which is the 
smallest size allowed outside of fixtures. Every length of Code 
wire has attached to it a label of the inspection department of 
the Fire Underwriters, certifying that the wire has been made in 
accordance with the rules laid do wnin the Code. Rubber- 
insulated wire must be used in all conduit systems, in wood or 
metal moulding and in knob and tube work. It is also fre¬ 
quently used for open work and is sometimes required by local 
rules. For damp places, rubber-insulated wire must be used. 
If rubber insulation is continually exposed to very high temper¬ 
atures, it deteriorates and becomes brittle. To ensure a long 
life, it has been found that the temperature should not exceed 
150° Fahr. for any considerable length of time. 

258. Weatherproof Insulation. Weatherproof wire (Fig. 
178-5), has an insulation consisting of three braids of fibrous 
yarn placed over the copper conductor, which is not tinned. 
During the process of manufacture the braids are thoroughly 
saturated with a waterproof compound. This wire is used 


256 


WIRES AND CABLES 


[chap. 14 


chiefly for outdoor service (Rule 12 ), and is not allowed for 
interior wiring except where corrosive vapors exist (Rule 26i). 
The advantages of this insulation are that it is cheap and dur¬ 
able except when exposed to high temperatures. The disad¬ 
vantage is that it is very inflammable. Slow-burning weather¬ 
proof wire is not as inflammable as weatherproof wire. The 
insulation consists of an inner braided covering which is weather¬ 
proof and an outer braided covering which is flameproof 
(Rule 55 ). This wire is not really fireproof, as the inside cov¬ 
ering will burn if subjected to enough heat. The outside cov¬ 
ering is filled with a flameproof paint, which will not easily 
carry a flame along the wire. Slow-burning weatherproof wire 
is allowed by the Code for use in open wiring in dry places 
(Rule 26 < 7 ), but it is not commonly used. Slow-burning wire 
(Fig. 178c), has three braids, all of which are filled with a flame¬ 
proof paint. This wire is somewhat like the old “ Under¬ 
writers’ ” wire (Rule 56). It is the wire most commonly used 
for open work in dry places, especially in factories. Fixture wire 
having slow-burning insulation is required where temperatures 
above 120° Fahr. exist, as in some designs of show-case fixtures 
(Rule 30c), and for indoor fixtures for large gas-filled incandes¬ 
cent lamps (Rule 35 d). None of the three kinds of insulation 
just mentioned are nearly so good as rubber as an insulator. 
In fact, when weatherproof or slow-burning wire is used, the 
porcelain supports are depended upon to properly insulate the 
wires. For this reason, these insulations cannot be used satis¬ 
factorily in damp places. The covering on the wire can be 
depended upon only to- prevent a short-circuit between wires 
and to reduce the danger of receiving a shock. All three kinds 
of wire are cheaper than rubber insulated wire. They are sold 
by the pound, whereas rubber wire is sold by the foot. 

259. Other Insulations. Conductors are sometimes insulated 
with paper impregnated with an insulating oil. Paper insulate*! 
cables must always be enclosed in a lead sheath to keep out 
moisture. They are used principally for high-voltage work in 
underground systems and are not recognized by the Code for 
ordinary low-voltage interior wiring. Varnished-cambric in¬ 
sulated cables have been used to some extent for interior 
wiring. This insulation consists of spirally wrapped layers 
of cotton tape which has been treated with an insulating var¬ 
nish. In the process of wrapping, the layers are coated with 


par. 260 ] 


MULTIPLE CONDUCTORS 


257 


a thick insulating compound to fill the spaces. Cable of this 
type is made with two braids similar to rubber cables. When so 
constructed it will withstand immersion in water and can be 
tested in the same manner as rubber insulation. It is not suit¬ 
able, however, for installation where it is permanently exposed 
to moisture, as the filling compound will gradually work out and 
the insulation be destroyed. There is also some tendency for 
the cable to dry out when exposed to a warm atmosphere. The 
cost of such a braid-covered cable is less than rubber insulation, 
but varnished-cambric cable is not yet recognized by the Code 
for general use in interior wiring. It is used extensively, how¬ 
ever, in power stations, especially for high-voltage wiring. By 
the addition of a lead sheath, the varnished-cambric cable can 
be used in wet places and so constructed is used to a considerable 
extent for underground systems in place of paper or rubber in¬ 
sulation. It costs less than rubber and more than paper insula¬ 
tion. 

260. Multiple Conductors. Frequently two or more wires 
are combined in one braid or lead sheath to form a multiple 
cable. Duplex or twin wires (Fig. 178e), consist of two rubber- 
insulated and single-braided conductors, laid parallel and cov¬ 
ered with a common braid. Wires of this kind occupy slightly 
less space than two single wires and also cost less. They are, 
therefore, very commonly used for branch circuits in lighting 
service. Triplex or three-conductor cables have three sep¬ 
arately insulated conductors enclosed in a common braid. 
These cables are used for small three-wire circuits and some¬ 
times for three-phase work. Flexible cords (Rule 51) consist 
usually of two separately insulated, extra flexible, stranded wires. 
The wires which make up the strand are very small, generally 
about No. 30. Rubber insulation is placed on each wire and 
sometimes another layer of rubber is placed over the conductors 
after they have been twisted together. Ordinary lamp cord 
has a cotton braid placed over the rubber insulation on each 
conductor. The two conductors are then twisted together 
(Fig. 179a). Lamp cord is allowed for use as a pendant or drop- 
fixture in dry places (Rule 32). Canvasite and brewery cords 
(Fig. 1796) have a weatherproof braid around each conductor 
and a second weatherproof braid around the two conductors 
after they have been twisted together. These cords are for use 
as pendants in damp places. Reinforced cord (Fig. 179c) is 


WIRES AND CABLES 


[CHAP. 14 


; 

258 

twisted lamp cord covered with a rubber jacket enclosing both 
conductors. A single cotton braid covers this rubber jacket. 
This cord is used for portables in dry places where not subjected 
to rough usage as in offices, dwellings, etc. (Rule 51/). For damp 
places, the outer braid is weatherproof (Reinforced cord, 
weatherproof) or two weatherproof braids are used (Packing¬ 
house cord). Armored cords have a flexible, galvanized steel 
armor similar to that used on the regular armored cable de¬ 
scribed in paragraph 238. The cord which is armored may be 
either regular lamp cord, or reinforced cord for dry places, or 


Close wind 


of Cotton T) i n /-v »• 



Fig. 179.— Flexible Cords. 

a. Lamp cord. b. Canvasite cord. c. Reinforced cord. 


weatherproof reinforced cord for damp places. Armored cords 
are used chiefly for portables subjected to rough usage, as in 
show windows. 

261. Carrying Capacity of Conductors. The current which in¬ 
sulated conductors will stand is limited to an amount which will 
not overheat the wires and damage the insulation. Rubber insu¬ 
lation will not withstand as high a temperature as weatherproof 
insulation without deteriorating and, hence, a lower current rat¬ 
ing is used for rubber wire. Table 36 gives the maximum current 
allowed by the Code for both types of insulation. The Code 
makes no distinction in carrying capacity of wires when used for 
alternating or direct current. For alternating currents, par- 
















par. 262 ] 


MULTIPLE CONDUCTORS 


259 


ticularly at 60 cycles, the apparent resistance is increased due 
to the skin effect.* This would cause the wires to run some¬ 
what hotter than when carrying direct current. According to 
the rules of the Code, the rating of the fuse protecting a wire 
must not be greater than the safe carrying capacity as given in 



Fig. 180 . —Cable Connectors. (Dossert.) 

a. Two-way connector, type A for light mechanical strains, b. Two-way 
connector, type B for heavy mechanical strains, c. Cable tap. 


Table 36. This makes it possible to carry a small overload on 
the wires, since enclosed fuses will carry 10 per cent excess cur¬ 
rent and open fuses about 25 per cent excess, without opening 
the circuits. Table 36 gives the maximum current which wires 
can carry, but in many cases the 
current carried must be made 
considerably less than the values 
given, to prevent excessive volt¬ 
age drop.f This matter is fully 
treated in Chapter 19. 

262. Splicing Wires and Cables. 

The Code requires that all taps 
and splices be made mechanic¬ 
ally secure and then soldered 
(Rule 16c). An exception to 
this is made where an approved 
connector is used. The common types of one make are shown 
in Fig. 180. There are several approved connectors for splicing 
fixture wire, one of which is shown in Fig. 181. Stranded wires 
may be spliced as shown in Fig. 182. A somewhat simpler 
* Paragraph 323. t Paragraph 320. 


Insulating Cap. Steel Screw 



Mou ldecC / r> , . 

Insulation Brass Bushing 

Fig. 181 . —Fixture Wire Con¬ 
nector. (USEM). 






























260 


WIRES AND CABLES 


[chap. 14 


form of splice can be used where the wires are not subjected to 
mechanical strains. Splices must be thoroughly soldered, using 
a non-corrosive soldering paste or fluid. So-called “ acid ’ 
(muriatic acid “ cut ” with zinc) should not be used on electrical 
work. When rubber-covered wires are spliced, the soldered 



Fig. 182.—Method of Splicing a Cable. 

The upper illustration shows strands laid out straight and centre core cut 
away. The next illustration shows half the remaining strands wrapped around 
the cable. The lower illustration shows the splice soldered and taped. 


connection must be covered with rubber compound tape to a 
thickness equal to the insulation on the wire and then covered 
with friction tape. Splices in weatherproof wires are covered 
with friction tape only. 












CHAPTER 15 


SWITCHES, CIRCUIT BREAKERS AND FUSES 

Knife Switches 

263. Construction. Knife switches consist of a blade hinged 
at one end and arranged to enter a forked terminal or jaw at the 
other. The general arrangement of knife switches of various 
sizes is shown in Figs. 183, 184 and 185. The blade is made of 


Blade 



Fig. 183.—Knife Switch. Back connected, double pole, single 

throw, 800 amperes, 250 volts. 

one or more copper bars, the cross-section depending upon the 
rated capacity of the switch. Large-capacity switches have 
multiple blades (Fig. 183). The clip and hinge are made 
somewhat similar and are composed of hard-rolled copper 
so that they will make good spring contact with the blade. 
At the hinge end, spring washers are used to secure proper 
contact. The switch must be mounted on an insulating base 
which may be a switchboard panel or an individual slab. For 

261 
































































































































262 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 


very small switches porcelain bases are used. For larger 
switches, usually above 30 amperes, slate or marble bases are 
generally used because porcelain in large slabs is difficult to 
make and is not strong mechanically. Switches for voltages 
higher than 3300 volts are generally mounted on porcelain insu¬ 
lators. The Code (Rule 65) specifies the general construction of 

all knife switches, as 
regards size of parts, 
spacing, etc. The 
connections to the 
switch may be either 
on the front or back, 
the kind used depend¬ 
ing upon the service 
intended. In back- 
connected switches (Fig. 183), the hinge and clip are connected 
to threaded copper studs which project through the base suf¬ 
ficiently to provide space for the connections. Switches for 
currents larger than 30 amperes must be provided with terminal 
lugs into which the wires are soldered (Rule Q5h). These lugs 
are clamped between nuts on the threaded studs. In some 
cases, instead of the studs, flat 
bars are used. The flat copper 
connections are bolted directly 
to these bars. Front-connected 
switches have an extension on 
each hinge and clip, arranged 
for connecting to the wires, with 
terminal lugs where required 
(Fig. 184). Switches of more 
than one pole (two-pole, three- 
pole, etc.) are built from single¬ 
pole switches by connecting 
the blades to an insulating cross-bar to which the handle is 
attached. Switches are made two-throw by providing a set of 
clips on either side of the hinge. Knife switches are made in 
sizes from 30 to 10,000 amperes or more. Fig. 185 shows a 
type of switch used in branch circuits for lighting panel boards. 
The Code requires that the rating of a switch be stamped on the 
blade. This includes the maximum current and the voltage. 
This does not necessarily mean that the switch will open this 



Fig. 185.—Fused Knife Switch 
for Lighting Panel Boards. 



Fig. 184.—Front-connected Knife Switch. 
Single pole, single throw, fused, 100 
amperes, 250 volts. 






























































par. 264] 


KNIFE SWITCHES 


263 


current when used on a circuit of the rated voltage. The voltage 
rating simply means that the break distance, that is the dis¬ 
tance between nearest points on the blade and clip of the same 
pole, and the spacing between poles, is in accordance with the 
values given in the Code (Rule 6 5k). With large capacity 
switches, particularly for 600 volts, it would not be safe to 
open heavy currents by means of an ordinary knife switch. 
Instead, either a circuit breaker or a quick-break switch must be 
used. 

264. Quick-break Switches are provided with an auxiliary 
blade which is independent of the handle (Fig. 186). When the 
main blade is first moved out 
of the clip, this auxiliary blade 
remains in the clip and keeps the 
circuit closed. Further motion 
of the main blade puts a tension 
on springs which connect the 
two parts of the blade. Finally 
a stop at the hinge end of the 
auxiliary blade prevents any 
further extension of the springs, 
and the blade is pulled out of 
the clip. The springs then give 
the blade a quick motion, thus breaking the circuit rapidly and 
reducing the arcing. Quick-break switches are used for the 
field circuits of a.c. generators and for breaking large currents 
at 250 volts and above. Quick-break switches must be used 
for switches designed for currents greater than 100 amperes at 
voltages higher than 250 (Rule 65&). 

265. Applications. For all circuits larger than about 20 
amperes’ capacity, knife switches (or sometimes circuit breakers) 
are used for the purpose of disconnecting a circuit or for trans¬ 
ferring it from one supply to another. On switchboards they 
are used for disconnecting generators and feeders. When the 
supply of electricity is from a central station, a service switch 
and fuse-cutout are always provided (Rule 24a). Switches are 
also used on panel boards for disconnecting branch circuits and 
for individual motor circuits, to entirely disconnect the motor 
and control apparatus from the line. Single-pole switches are 
allowed only in special cases. Usually the switch must dis¬ 
connect all wires of a circuit (Rule 24c). Single-throw switches 



Fig. 186.—Quick-break Knife 
Switch. 












264 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 


are placed handle up so that gravity will tend to open the 
switch. Wherever possible, the line side of the circuit is wired 
to the top terminals of the switch, so that the blades will be 
“ dead ” when the switch is open. Double-throw switches may 
be mounted horizontally or vertically. If mounted vertically, a 
stop must be provided to prevent the switch blades falling into 
the lower clips. Knife switches are used for all d.c. voltages. 
For alternating current they are commonly used up to 600 
volts and in special cases for higher voltages. Oil switches are 
better for alternating current circuits above 250 volts,* 



Rotary Snap Switches 

266. Construction. Snap switches are used for controlling 
small currents. They consist of pivoted insulated blades which 

make contact with the 
terminals to which the 
circuit wires are at¬ 
tached. The blades are 
moved by means of a 
handle, through a 
spring and cam motion, 
which does not allow 
the blades to move 
until the handle has 
been twisted nearly a 
quarter turn. When 
this is done, the blades 
are released and are given a quick motion to open or close the 
switch. The ordinary form of snap switch for mounting on the 
surface of a wall is shown in Fig. 187. The switches are also 
mounted in an enclosing porcelain base and are then used for 
concealed work, with only the handle projecting. Such switches 
are called rotary flush switches (Fig. 188). Switches are made 
both single- and double-pole in capacities of 5, 10, 20 and 30 
amperes. The two larger sizes are not used as much as the others. 
The ordinary switches are not designed for more than 250 volts, 
although special switches for 500 volts may be obtained. 

267. Special Snap Switches. Besides the ordinary single- or 
double-pole switches which are used to open one or both lines 

* See paragraph 276. 


Fig. 187. —Rotary Snap Switch. 
(Double pole.) 


































par. 268 ] 


SNAP SWITCHES 


265 



of a two-wire circuit, there are a number of special snap switches 
in common use. These are the same size as the regular switches, 
the difference being in the arrangement of the parts. Three- 
way switches have three terminals and are arranged so that the 
switch blade always makes contact with one or the other of two 
of the terminals. They are used to control lamps from two 
points as shown in Fig. 225. Since these switches control only 
one side of the circuit, 
they must be con¬ 
sidered as single pole. 

When lamps are to be 
controlled from more 
than two points, a 
four-way switch must 
be used at the inter¬ 
mediate points (Fig. 

225). These switches 
have four terminals 
and two switch blades. 

The terminals are 
connected together in 
different ways for 
each motion of the 
handle. Two-point 
electrolier switches 
have three terminals 

and a single blade. They are used to light either or both 
groups of lamps or to extinguish them all. Three-point 
electrolier switches have four terminals and a single blade. 
They are used to control lights in three sections. Electrolier 
switches are used for large fixtures or “ electroliers ” to give 
control of the lamps in groups. 

268. Switch Fittings. Surface switches, when used with 
exposed wiring, are supported away from the wall either by 
porcelain knobs or by porcelain switch blocks or sub-bases which 
have grooves for the entrance of the wires to the base of the 
switch (Fig. 189). For moulding or concealed work, similar 
porcelain bases are used. When flush switches are used, they 
must be enclosed in a metal outlet box, whether they are used 
with conduit systems or not (Rule 24d). Usually regular steel 
outlet boxes are employed, with a cover having a rectangular 



Fig. 188. —Rotary Flush Switch. (Single 

pole.) 

a. Switch plate. b. Switch with plate and 
operating handle removed. 
































































































266 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 


opening to fit the switch (Fig. 122c). For flush switches, metal 
switch plates (Fig. 188a), are used to cover the mechanism and 
the outlet box. These are about | in. thick and are finished to 
match the hardware in the room. 

269. Applications. Snap switches are 
generally used for controlling branch 
lighting circuits and small motors. 
They are preferable to knife switches 
for circuits up to 20 amperes and 250 
volts because they give greater protec¬ 
tion to the user and can be installed in 
places where knife switches would not 
be suitable. The rules under which 
single- and double-pole switches are 
installed are given in paragraph 301. Double-pole switches 
cost about 30 per cent more than single-pole. Surface switches 
are used with open or moulding wiring, and also in cheap 
installations of knob and tube wiring. For concealed work, 
the flush type switch is sometimes used, 
but more commonly a push-button 
switch is employed. 

Push-button Switches^ 

270. Construction. Push-button 
switches are used only for small cur¬ 
rents. They contain one or more con¬ 
tact blades which are given a rocking 
motion by the buttons, instead of the 
rotary motion as in snap switches. 

By means of a spring and cam mech¬ 
anism similar to that on the rotary 
snap switches, the blades make con¬ 
tact with clips or jaws attached to the 
terminals. The blades, therefore, open 
and close with a quick motion, which 
assists in extinguishing the arc. Push¬ 
button switches are arranged in a por¬ 
celain block which encloses the live contacts. They are in¬ 
tended principally for flush mounting (Fig. 190). Switches 
are made in 5- and 10-ampere sizes and for voltages up to 250 
volts. Special switches, such as three-way and four-way, 



Fig. 190.—Push-button 
Switch. With por¬ 
tion of porcelain cut 
away to show mech¬ 
anism. (Double 
pole.) 



Fig. 189.—Porcelain 
[Switch Base. 













































































PAR. 271 ] 


PUSH-BUTTON SWITCHES 


267 


are also made. The operation of these is the same as for 
the rotary switches described in paragraph 267. A momentary- 
contact switch is also made for operating remote-control switches. 
The mechanism is similar to the regular push-button switches, 
except that the contact is made only as long as the button is 
pressed. Pressing the other button makes connection to an¬ 
other circuit. Both buttons cannot be pressed at the same time. 
When a button is released, the contact is opened with a quick 
break. Automatic door switches of the push-button type are 
also made. They have only one button and are installed in 
the jamb of the door. One type turns the 
lamps on when the door is closed and the other 
when the door is opened. Push-button switches 
are mounted in steel outlet boxes with suitable 
covers (Fig. 122c). Switch plates similar to 
those used for rotary flush switches are em¬ 
ployed. Sometimes push-button switches are 
used on exposed conduit work. In such cases 
they are mounted in a cast-iron box (Fig. 130c). 

Pendant switches (Fig. 191) are a modified form 
of push-button switch arranged to hang from a 
pendant cord. They are used to control ceiling 
fixtures which are out of reach, and are cheaper than a regular 
wall switch. 

271. Applications. Push-button switches are used more 
commonly for controlling branch lighting circuits than snap 
switches. The same rules apply with regard to the use of single¬ 
pole switches. For the best service, double-pole switches should 
be used. The cost of these is about 25 per cent more than single 
pole. 

272. Remote-control Switches. These are magnetically 
operated switches. The contacts are opened and closed by elec¬ 
tromagnets which are controlled (usually at a considerable dis¬ 
tance) by means of a small two-throw knife switch or a momen¬ 
tary-contact push-button switch. Fig. 192 shows the method 
of operation. Remote-control switches are used with large fix¬ 
tures to simplify the wiring since the point of control is usually 
a considerable distance away. If hand control is used, the main 
circuit, which must be of large size, would have to be run to 
the control point. If a remote control switch is used, it can be 
located at any convenient place near the fixture and only three 



Pendant 

Switch. 



























268 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 

small wires need be run to the control point. This usually 
results in a considerable saving in copper for very large fixtures. 
The use of this switch has the further advantage that a push¬ 
button switch can be used for the control of large currents, 
whereas with hand control a knife switch would have to be used. 
This is more difficult to conceal, and more dangerous for an 
unskilled person to operate. Remote control switches are also 
sometimes used in connection with burglar alarms to throw on a 
part or all the lights in a residence, by the operation of a control 



Fig.' 192. —Remote-control Switch. 

a. Front-connected switch with three-wire mains and four two-wire branches. 
b. Diagram of connections. Switches (s) are momentary-contact push-button 
switches which can be located at any convenient points. As shown, the switch 
could be controlled from two points. The trip circuit is X and closing circuit Z. 


switch. Small motors, which do not require any starting rheo¬ 
stat, can also be started and stopped at a distance by this type 
of switch. 

Circuit Breakers—Air-break Type 
« 

273. Construction. The purpose of a circuit breaker is to 
open a circuit automatically whenever conditions on the circuit 
which it protects have become abnormal. Usually they are 
employed to open the circuit when the current exceeds a safe 
value, but they are sometimes used for other purposes, such as 
to open on reversal of current flow, failure of voltage, etc. 

























































par. 273] 


AIR-BREAK CIRCUIT BREAKERS 


239 


The contacts are held closed by a latch which is tripped by the 
armature of an electromagnet connected into the circuit. The 
spring action of the movable contacts, and in some cases addi¬ 
tional springs, cause the contacts to open quickly. Ordinary 
overload circuit breakers (Fig. 193), which protect against 
excessive current, are most commonly employed. They are 
used for all d.c. voltages and when suitably designed may be 




Fig. 193.—Circuit Breaker. Single pole, 600 volts, 400 amperes, 

plain overload. 

Current enters at stud (1) and passes through laminated brush (2), lower 
contact block (8) and coil (4) and to stud (3). If the current exceeds the value 
for which the breaker is set, armature (5) is lifted by current in coil (4), thus 
striking arm (9) and unlatching the toggle. The breaker thus opens, taking the 
position shown in dotted lines. As the breaker opens, the brush leaves the con¬ 
tact block first, then (7) opens and finally the circuit is broken by the carbon 
contacts (6) so as to prevent burning of the main contacts. Breaker can be 
set to trip at a lower current by raising (5) . 

used on alternating current. It is more common, however, 
to use oil circuit breakers* for alternating current. Circuit 
breakers are rated at the current which they will carry con¬ 
tinuously without overheating and at the maximum voltage 
for which they should be used. The setting of a breaker can be 
adjusted to suit requirements, the range of setting for one 
standard line of breakers being from 80 to 160 per cent of the 
rating. Circuit breakers are made in many sizes to suit all 

* See paragraph 276. 











































































270 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 


requirements from 25 amperes to 10,000 amperes or more. 
They are made one-, two-, three- or four-pole in the smaller sizes. 
Very large breakers are usually single pole. Multiple breakers 
have trip coils on all poles, the tripping of one pole opening all. 
In some cases, the breakers are so designed that they cannot be 
held closed if an overload exists on the circuit. These are 
required where no knife switch is used. The ordinary overload 
circuit breaker acts almost instantaneously and, hence, is more 
sensitive to momentary heavy overloads than is an enclosed fuse. 
For example, a 15-ampere enclosed fuse will stand a 50 per cent 
overload for about^l minute, without blowing. A circuit breaker set 
at 10 amperes would trip as soon as the current reached this value. 
In fact, a circuit breaker must be set very much higher than the 
fuse rating to make the fuse blow before the breaker opens on a 
sudden over load. The amount of this increase would vary with 
the design of the breaker, but in one case it was found that the 
breaker had to be set at 90 amperes before a 15-ampere fuse 
would blow when the current was thrown on suddenly. 

274. Applications. Circuit breakers must be used for circuits 
carrying currents larger than the approved sizes of fuses. The 
largest sizes of approved enclosed fuses are 600 amperes up to 250 
volts and 400 amperes from 250 to 600 volts. Under certain condi¬ 
tions, the Code allows the installation of enclosed fuses in multiple 
for currents larger than the capacity of a single fuse (Rule 23e).* 
Circuit breakers are commonly used, even in small capacities, 
for motor circuits and similar places subject to frequent over¬ 
loads. When a circuit is opened by means of a breaker, it may 
be restored more quickly and with practically no expense as 
compared with a fuse. The first cost of a circuit breaker is, of 
course, much greater than an equipment of enclosed fuses. Cir¬ 
cuit breakers should always be used on switchboards to protect 
motor feeders, and in some cases they can be used to advantage 
even on lighting feeders. It is also frequently good practice to 
use them to protect individual motors, especially if of large size, 
although in this case fuses also must be installed, unless the 
equipment is subject to competent supervision (Rule 23/). 
When a breaker is used, the fuses would be made large enough 
to blow only if the breaker failed to operate. Circuit breakers 
should never be installed where they are exposed directly to the 
severe conditions which exist in cement or flour mills, plaster or 

* See paragraph 281. 


par. 275] 


OIL CIRCUIT BREAKERS 


271 


furniture factories and similar places. In such cases, oil cir¬ 
cuit breakers or enclosed fuses are better. When installed in 
places where considerable dust or dirt may accumulate, the 
breakers should be cleaned regularly to keep the contacts in 
good condition and prevent overheating. In many industrial 
establishments, it is desirable to enclose the breaker in a steel 
cabinet for protection. Circuit breakers must not be set more 
than 30 per cent above the allowable carrying capacity of the 
wire, as given in Table 36, unless a fuse is used to protect the 
wire (Rule 23e). 

Oil Circuit Breakers 

275. Construction. With the air-break type of circuit 
breakers, just described, the arc formed when the apparatus 
opens is freely exposed in the air, and under some conditions 
this may result in a considerable fire hazard. The type of 
breaker to be described has all the current-carrying contacts 
immersed in heavy mineral oil, and the arc is broken at a con¬ 
siderable depth below the surface of this oil. As a result of this, 
the insulation of the breaker is very much improved, so that it 
can be made for very high voltages, and the different poles can 
be located close to each other without the danger of flashing 
across which exists with the air-type breaker. Since the arc is 
entirely beneath the surface of the oil, the danger from fire is 
practically eliminated. At the same time, the contacts and 
mechanism are easily protected from dust and dirt. The gen¬ 
eral arrangement of an oil circuit breaker is shown in Fig. 194. 
The arc which is formed when the contacts open is extinguished 
by the oil surrounding them. As used on a.c. systems, the 
arc is extinguished when the current is at the zero point of 
the wave. Oil circuit breakers are not satisfactory for use 
on direct current. They may be of the plain overload type, 
or by the use of relays may be made to open the circuit 
under any desired set of conditions. The breakers are made 
with one, two, three or four poles to suit various kinds of cir¬ 
cuits. For small capacities and relatively low voltages, the 
contacts for all the poles are contained in the same tank. For 
larger breakers, especially for high-voltage service, each pole is 
located in a separate tank. Breakers of the overload type are 
opened by trip coils, which are either operated directly by the 
line current or through the medium of current transformers and 


272 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 


relays. While ordinary overload circuit breakers operate very 
much like the air-break type when subjected to heavy momentary 
overloads, it is easily possible by means of relays to retard the 
action of the oil circuit breaker to any reasonable extent, so that 
its action on overloads would resemble a fuse. This is an ad¬ 
vantage in many cases where sudden heavy overloads occur, 
but where they last for so short a time that they are not dan¬ 
gerous and, hence, the 
circuit need not be 
opened. When breakers 
are so adjusted they are 
said to have a “ time 
limit ” characteristic. 
Oil circuit breakers are 
rated at the amperes 
which they will carry 
continuously and the 
maximum voltage for 
which they are safe. 
They can, of course, be 
used for any voltage less 
than this. Oil circuit 
breakers are also rated 
according to their break¬ 
ing capacity; that is, a 
breaker connected to a 
large system has to be 
capable of interrupting 
a greater flow of power 
than on a small system. 
The required breaking 
capacity is, therefore, de¬ 
termined by the capacity 
in generators, transformers, etc., which is connected to the same 
busbars as the breaker. The manufacturer of the circuit 
breaker should be consulted where there is any doubt as to the 
size of breaker to be used. The trip coils can be set to operate 
at loads above and below the rating, a typical range being from 
80 per cent to 160 per cent of rating. Some types of oil circuit 
breakers will not trip out upon an overload if the operating han¬ 
dle is held in the closed position. The best type of breaker is so 




Fig. 194. —Oil Switch. (Three pole.) 

The same device is also provided with trip 
coils to open the circuit automatically on over¬ 
load. This type of switch is adapted for 
industrial applications for the control of in¬ 
duction motors at voltages not exceeding 
2500 volts. 






























































































par. 276] 


OPEN FUSES 


273 


arranged that it will open immediately if closed when an over¬ 
load exists on the line. 

276. Applications. The Code rules regarding the use of air- 
type breakers apply also to oil circuit breakers. The advantages 
of the oil circuit breaker over the air-break type are: the 
absence of exposed arcing; protection from dust, dirt, etc.; 
greater compactness; better insulation and protection of live 
parts so that danger of shock is eliminated. For these reasons, 
oil circuit breakers are very commonly used for a.c. work up to 
550 volts and are always used on higher voltages, except for 
very small capacity circuits. Oil circuit breakers are very com¬ 
monly used for motor circuits, both on the switchboard and for 
the individual motors. Sometimes when enclosed fuses are 
used to protect the motor an oil switch is used for control. An 
oil switch is the same as a breaker, except that it does not have 
any automatic tripping mechanism, and must be operated by 
hand to open the circuit. Fig. 214 shows oil circuit breakers 
installed on a switchboard. 


Open Fuses 


277. Construction. The open fuse is the simplest device used 


for the protection of a circuit, 
made usually of an easily melted 
alloy, and so small in cross- 
section that it will melt before 
the line wires are overheated. 
At one time fuse wire, an alloy of 
lead,;tin or other metals,was used. 
A short piece of this wire was 
clamped between two contact 
blocks which were mounted on 
an insulating base. It was soon 
found, however, that fuse wire 
was very unreliable, since drafts 
of air and variations in the 
lengths of wire used affected the 
melting-point of the fuse. The 
next step was to attach a definite 


It consists of a wire or strip 



Fig. 195.—Link Fuses. 

a. Fuse-wire type. b. Fuse-strip 
type. c. Copper link. 


length of fuse wire to copper terminals, thus forming a link fuse 
(Fig. 195). Later, links were made from strips of copper, 
aluminum, etc. For small sizes, the lead alloy link fuse is 


































































































































274 SWITCHES, CIRCUIT-BREAKERS AND FUSES [CHAP. 15 


approved. For larger currents, copper or aluminum fuses are 
generally preferred. Link fuses will carry a current about 25 
per cent above their rating without melting (Rule 685). 

278. Applications. Link fuses are used principally on switch¬ 
boards or panel boards. They are seldom used in individual 
cutouts. The only advantage they have is the low cost. The 
disadvantages are that they are unreliable, cause a considerable 
flame when they blow, and soon disfigure the panel upon which 
they are mounted. They also may cause injury to a person 
nearby when they blow. Link fuses must always be enclosed 
in a metal box or cabinet, except when mounted on a switch¬ 
board (Rule 23c). 


Enclosed Fuses 

279. Plug Fuses. The earliest form of enclosed fuse was the 
Edison plug fuse. The type now in use (Fig. 196) is only slightly 

modified from the original design. It 
consists of a length of lead alloy fuse wire 
mounted in a porcelain cup with a mica 
cover. The contacts are the same as the 
medium-size Edison base for incandescent 
lamps. This plug fuse is made in various 
sizes up to 30 amperes for use on 125- 
volt circuits or 250-125-volt three-wire 
systems. It is not very accurate nor 
reliable because of the short length of fuse 
wire. It is also rather expensive, although 
considerably cheaper than the cartridge 
type of enclosed fuse. Plug fuses are used very extensively both 
in panel boards for branch circuits, and in individual cutouts of 
small capacity. The ordinary Edison plug fuse (Fig. 196) is 
not approved for 250-volt service. For this a regular cartridge 
fuse of the 0-30-ampere size is used. This is enclosed in a por¬ 
celain shell, which has standard plug fuse threads so that it can 
be used in a regular cutout. The maximum size for this type 
of fuse is 30 amperes. There is also a large size 250-volt plug fuse 
which is approved in sizes up to 60 amperes. This uses the 
regular 31-60-ampere size of cartridge fuse and is the same as the 
one just described except that it is larger. These plugs are used 
in large-size Edison plug cutouts. 



Fig. 196.—Edison 
Plug Fuse. 






































par. 280] 


ENCLOSED FUSES 


275 


280. Cartridge Fuses. The most common type of enclosed 
fuse is the cartridge fuse. The current passes through a fusible 
link held in the centre of a heavy fibre tube (Fig. 197). This 
link is made of aluminum or zinc and is soldered to copper ter¬ 
minals which serve to make contact with the circuit. The shape 



Indicator wire 


” link 


Terminal 


Fig. 197.—Sectional View of Cartridge Fuse. 


• Terminal 


of these links varies with the size of the fuse and with the man¬ 
ufacturer. The fuse link is surrounded by a filling which con¬ 
sists of porous, non-conducting material. Some manufacturers 
use the material in the form of a fine powder and others use a 
granular filling. When the fuse “ blows ” some of the metal of 
the link is vaporized and 
rendered conducting, so 
that an arc tends to form. 

The filling must quickly 
absorb this vapor and 
cool or condense it so 
that it is rendered non¬ 
conducting, and the arc 
thereby extinguished. 

Whiting, rotten stone, 
finely divided asbestos 
and other materials of a 
similar nature are used 
for filling. With the 
larger sizes of fuses, vents 

are provided in the end caps, to reduce the internal pressure 
when the fuse blows. For fuses up to 60 amperes* contact 
with the circuit is made by clips which grip the end caps 
(Fig. 198a). For larger sizes, the knife-blade contact is used 
(Fig. 1986). One difficulty with an enclosed fuse is to know 

* National Electrical Code standard 



Fig. 198.—Enclosed Fuses. (Cartridge 
type.) 

a. Ferrule contacts, b. Knife-blade con¬ 


tacts. 



























































276 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 


when it is blown. There are several types of indicators used 
for this purpose. In one of these, a small wire is attached to 
the terminals and carried outside the tube near the centre 
(Fig. 197). This wire is covered by the paper label, so that 
when the fuse blows the wire melts and leaves a burned spot 
in the paper. Another type has a portion of the end cap which 
moves out when the fuse blows. 

281. Standard Sizes. The construction of approved en¬ 
closed fuses is carefully specified in the Code (Rule 68). There 
are two classes of fuses, one for circuits up to 250 volts and the 
other for 250- to 600-volt circuits. The 600-volt fuses are longer 
than the 250-volt fuses and will not fit the same cutouts. The 
dimensions of the various capacities of fuses are such that only a 
limited range of sizes will fit a particular size of cutout. The 
range is as follows: 


Cut-out. 

Sizes of Fuses. 

250-volt. 

600-volt. 

30 amperes 

3- 30 amperes 

3- 30 amperes 

^ 60 

35- 60 / 

35- 60 

100 

65-100 / 

65-100 

200 

110-200/ 

110-220 

400 

225-400 

225-400 

600 

450-600 
• / 

not approved 


From five to ten intermediate capacities of fuses can be obtained 
in each group. When fuses larger than those listed above are 
required, the Code allows the use of two or more fuses in mul¬ 
tiple (Rule 23e), provided the fuses are all of the same size and 
the fuse clips are all mounted on a single busbar. This arrange¬ 
ment is not recommended, however, because of the difficulty in 
making each fuse take its share of the load. Unless care is 
taken to make good connection with each fuse, the current will 
be divided unevenly and the fuses will blow even when no over¬ 
load exists. This arrangement is not allowed for motor cir¬ 
cuits. 

282. Action on Overloads. An enclosed fuse has a consider¬ 
able <£ time limit ” feature. lhat is, an overload which occurs 










par. 283] 


ENCLOSED FUSES 


277 


for only an instant would not blow the fuse, whereas if the over¬ 
load continued for several minutes, the fuse would have time to 
heat up, and it would then blow. The action is, therefore, dif¬ 
ferent from the ordinary overload circuit breaker, as was ex¬ 
plained in paragraph 273. The rating of an enclosed fuse is the 
current value marked on the label. The construction rules 
(Nos. 68 h and i) require that enclosed fuses shall carry indefi¬ 
nitely, under ordinary conditions, a current 10 per cent greater 
than this rating, and that with a current 25 per cent greater they 
shall open the circuit without exceeding a safe temperature. It 
is also required that with an overload of 50 per cent the fuses 
shall blow within the time specified below: 


1 minute 

2 minutes 


0- 30 amperes 


31- 60 
61-100 
101-200 
201-400 
401-600 


4 

6 

12 

15 


The time given above is with the fuse cold at the start of the 
test. Under usual conditions, the fuse would be warm, from the 
regular current, so that the time to open an overload would be 
shorter than is indicated. 

283. Refilled Fuses. Cartridge fuses, 65 amperes and larger, 
are refilled by the manufacturers at a price from one-half to one- 
third the price for new fuses. When refilled in this way they 
are guaranteed to be as accurate as new fuses and are approved 
by the Code. There are a number of so-called renewable fuses 
on the market, but none of them are at present (1916) approved 
by the Code. These fuses are made substantially like the stand¬ 
ard cartridge fuse, except that the caps are easily removable 
for the purpose of inserting the fusible link and filling. From 
the standpoint of the Underwriters, the objection to the use of 
these fuses is that, in unskilled hands, the refilling may not be 
done properly, or too large a link may be used. It is probable 
that before long the use of these fuses will be cdlowed under cer¬ 
tain restrictions. 

284. Enclosed Fuse Cutouts. In order that the fuses may be 
easily replaced in the circuit, cutouts are provided. Plug 
fuse cutouts (Fig. 199), are made single, double and triple pole, 
for branch and main-line work. Contact is made with the 


278 SWITCHES, CIRCUIT BREAKERS AND FUSES [CHAP. 15 

threaded shell into which the plug is screwed and also with an 
insulated stud at the bottom of the shell. Similar cutouts are 

frequently used on panel 
boards. Enclosed fuse cut¬ 
outs (Fig. 200) are made 
single, double and triple pole 
for 250 volts in sizes up to 
100 amperes. Larger sizes 
are single pole only. For 
600 volts, only single-pole 
cutouts are usual. The 
method of making connec¬ 
tion with the fuse is ap¬ 
parent from the illustra¬ 
tions. Where enclosed 
fuses are used on panel boards or switchboards, the same 
style of contact clips is used, the terminals being modified to 
suit the requirements and as¬ 
sembled directly on the slate or 
marble panel. 

286. Applications. Where a 
fuse is to be used to protect a cir¬ 
cuit, the enclosed type is the 
best. Link fuses are allowed 
only under special restrictions. 

They are also unreliable in action. 

The plug fuse is satisfactory for 
small-capacity circuits, but, in 
general, the cartridge fuse is 
most commonly used. For mod¬ 
erate sizes, approved enclosed 
fuses are reliable and entirely 
satisfactory. For large circuits, 
the cost of the fuse and the 
drop in voltage is considerable. 

Furthermore, when a large 

number of fuses are mounted on Single-pole mam-lme cutout, 
a switchboard or panel board, 

there is difficulty in quickly locating a blown fuse and in replac¬ 
ing it. This may result in long delay in restoring the service. 
One especial advantage of fuses is that they will not blow on 



Fig. 200. —Enclosed Fuse 
Cutouts. 

a. Double-pole branch cutout, b. 



Fig. 199.—Plug Fuse Cutouts. 

a. Double-pole branch cutout, b. Triple¬ 
pole main-line cutout. 
















































par. 285] 


ENCLOSED FUSES 


279 


heavy momentary overloads, which might trip a circuit breaker 
unless it was set so high that it would not give proper protection 
to the circuit against moderate overloads lasting a considerable 
time. Momentary overloads, even if quite heavy, usually do 
no harm to a motor or other apparatus, as there is not time 
enough for the machine to overheat. Hence it is desirable not 
to open the circuit in such cases. On the other hand, a mod¬ 
erate overload, if continued for a considerable time, will finally 
overheat the machine, and, therefore, the circuit should be 
opened before this occurs. In general, it may be stated that 
fuses are best adapted for lighting circuits, except for very large 
currents and for small motors such as would be used for indi¬ 
vidual machine tool drive, or for ordinary group drive. For 
large motors, or where the duty is severe, as on cranes, hoists, 
bending rolls, etc., circuit breakers would be used. Fuses must 
also be used in such cases (Rule 23/), in addition to the circuit 
breaker. The fuse protects the motor against overheating and 
the circuit breaker takes care of excessive momentary over¬ 
loads. In switchboards, or where the apparatus is under proper 
supervision, both fuses and circuit breakers are not required 
on the same circuit. 


CHAPTER 16 


SOCKETS AND RECEPTACLES 

286. The purpose of a socket or receptacle is to provide means 
by which an incandescent lamp may be readily connected to a 
circuit and replaced when burned out. The sockets and recep¬ 
tacles usually employed are arranged to take lamps with Edison 
bases of the three sizes previously mentioned.* The construc¬ 
tion of sockets and receptacles is carefully specified by the Code, 
and only devices which meet these specifications are approved 
for use. Sockets are designed to screw on to a pipe or to be 
suspended by a flexible cord. Receptacles are designed to 
attach to a wall or other flat surface such as the cover of an out¬ 
let box, etc. 

287. Sockets. Key sockets are used where it is desired to 
control the lamp at the socket (Fig. 201a). Usually only one 


a & 

Fig. 201.—Socket Bodies. For brass shell sockets. (See Fig. 202 
for illustration of shell and cap.) 
a. Key socket, b. Keyless socket, c. Pull socket. 

side of the circuit is opened at the key, but double-pole sockets 
which open both sides of the circuit are made by some manu¬ 
facturers. Pull sockets (Fig. 201c), have a rotary contact device 
which is operated by a chain. These are convenient for use 
where the socket is out of reach. They' cost nearly twice as 
much as the ordinary key socket. Keyless sockets (Fig. 201 ; ) 

* See paragraph 11. 

280 




























































par. 287] 


SOCKETS 


281 


are similar in construction to the key sockets already described 
with the key and contacts omitted. They would be used on 
fixtures where the lamps are controlled from separate switches. 
Keyless sockets cost about 10 per cent less than key sockets. 
Sockets for ordinary interior 
wiring have a brass shell and 
a cap which can be finished 
to match the decorations in 
the room (Fig. 202a). Porce¬ 
lain sockets (Fig. 2026) have 
a porcelain shell, a metal or 
porcelain cap and a porcelain 
insulated key if any is used. 

They are liable to breakage 
and are, therefore, not ap¬ 
proved where subject to rough 
usage. They are used in 
bathrooms and basements 
where there would be danger 
of a shock from a brass-shell socket. Sockets may be obtained 
with caps threaded to fit l, l, f or ^-in. iron pipe. Where they 
are hung from a flexible cord, not less than a f-in. size socket 
should be used and the opening provided with an insulating 




Fig. 202.—Sockets. 

a. Brass shell socket with cap re¬ 
moved to show projections which 
serve to clamp the two parts together. 
b. Porcelain socket for pendant cord. 



Fig. 203. —Method of Tying an “ Underwriters’ ” Knot. 

The first fold should be carried behind the straight wire. 


bushing (Rule 3Id). The cord must have a knot inside the 
socket cap to support the weight of the lamp and reflector so 
that there will be no danger of pulling the wires out of the con¬ 
tact clips. Fig. 203 shows the method of tying this knot. 








































































J 


282 


SOCKETS AND RECEPTACLES [CHAP. 16 


Weatherproof sockets (Fig. 204) are made with aluminum, 
porcelain and moulded composition shells. These sockets are 
keyless. The aluminum and porcelain-shell sockets can be 
obtained with caps threaded for f or 5 -in. iron pipe sizes. 

The porcelain and moulded 
composition sockets can be ob¬ 
tained with the connection wires 
directly attached, ready for 
fastening to the branch circuit. 
In this case, the socket is sup¬ 
ported by the lead wires. 
Mogul sockets are used for 
large tungsten lamps having 
mogul bases. These sockets are 
keyless and have metal or por¬ 
celain shells. They are also 
made in the weatherproof form, 
with porcelain shells. Cande¬ 
labra and miniature sockets are 
made both key and keyless (the latter most commonly used), 
with metal or porcelain shells. They are also made in the 
pull type. Miniature sockets are allowed only for decorative 
lighting systems, Christmas tree lighting outfits, and sim- 



Fig. 204.—Weatherproof 
Sockets. 

a. Aluminum shell socket. b. 
Moulded mica socket. For voltages 
up to 600 volts. 



Fig. 205.—Receptacles. 

a. Key, cleat receptacle, b. Keyless, porcelain receptacle, c. Sign recen- 
tacle. For voltages not exceeding 250 volts. 


ilar purposes. Receptacles are made in a large variety of 
styles. One style consists essentially of a regular brass socket 
shell and body mounted in a flat base (instead of a cap) so that 
the device can be placed on a flat surface or on an outlet box'. 


« 


































































































































































































par. 288] 


RECEPTACLES 


283 


This style can, therefore, be of the key, keyless, or pull type 
(Fig. 205a). Another style is made of a single piece of porcelain 
in which the contacts are mounted. The style shown in Fig. 
205c is for mounting in the cover of an outlet box or in an open¬ 
ing in a sheet metal sign. Another style (Fig. 2055) is mounted 
on the surface of a wall. 

288. Rating of Sockets and Receptacles. Sockets and re¬ 
ceptacles which are approved by the Code must be stamped 
with the maximum voltage and watts for which they may be 
used. The rating thus marked is in accordance with the fol¬ 
lowing classifications (Rule 72a): 


Ratings of Sockets and Receptacles 


Class. 

Key. 

Keyless. 

Watts. 

Volts. 

Max. 

Amp. 

Watts. 

Volts. 

Max. 

Amp. 

Candelabra base.. 

75 

125 

0.75 

75 

125 

1 

Medium base.... 

250 

250 

2.5 

660 

250 

6 

Medium base (a). 

660 

250 

6 

660 

600 


Mogul base. 

• • . • 

.... 


1500 

250 



(a) This rating allowed only where switch mechanism gives a quick “ make” 
and “ break ” action. 


In explanation of note (a) it may be said that the ordinary 
key socket or receptacle has a mechanism which closes or 
“ makes ” the circuit 
slowty but gives a quick 
“ break.” This type 
would, therefore, be 
rated at 250 watts. 

The 660-watt socket 
is only slightly more 
expensive. 

289. Rosettes. Where 
an ordinary flexible 
cord drop is used (Fig. •" 

24), a rosette is provided to support the cord and socket. 
Devices similar to Fig. 206a are used for cleat work. Fig. 2066 
shows a very satisfactory style for concealed work. This 



Fig. 206.—Rosettes. 

a. Cleat rosette, b. Concealed rosette for 
use w T ith outlet box. 


































284 


SOCKETS AND RECEPTACLES 


[chap. 16 


rosette attaches directly to the outlet box. In all these devices 
the cord passes through a central hole and is anchored by a knot 
(Fig. 203), to relieve connections of any strain. The cord is 
then attached to contacts inside the porcelain body. Rosettes 
are usually made fuseless, as fuses are not, in general, allowed in 
rosettes (Rule 23d). For open wiring in large mills, the Code 
allows the use of link-fuse rosettes for 125 volts and enclosed 
fuse rosettes for 250 volts. The fuses must not be larger than 
3 amperes and the fuse in the branch circuit not over 25 amperes. 

290. Plug Receptacles. For attaching portable apparatus, 
such as heaters, fans, etc., plug receptacles are used. Th$y may 


Brass 

Plate 




Fig. 207.—Wall Receptacle 


and Plug. 


For flush mounting in a steel outlet box. 
View shows porcelain cut away to show con¬ 
tacts. The centre prong on plug enters first 
and uncovers the opening in the plate for the 
contacts to enter. 



Fig. 208— Floor Box. 


be either of the wall or the floor type. Wall receptacles are 
generally of the flush type and take the same size of outlet box 
as push-button switches. T he usual type carries spring clip 
contacts arranged to engage corresponding contact posts on 
an attachment plug which is connected to the flexible cord (Fig. 
207). One style of plug receptacle contains regular Edison 
lamp base contacts. In this a screw plug is used. Floor 
receptacles (big. 208) are similar to the wall receptacles, as 
far as the electrical parts are concerned, but the outlet box 
must be made waterproof. 


















































































* 


CHAPTER 17 

PANEL BOARDS AND SWITCHBOARDS 


291. Since the Code requires that for branch lighting circuits, 
not more than 660 watts (or under special conditions 1320 watts) 
can be dependent upon one 


Pus Bars 


cutout (Rule 23d), it is 
apparent that for all instal- 
lations, except the very 
smallest, there must be a 
considerable number of 
branch circuits, each hav¬ 
ing separate fuses. With 
motors, the same holds 
true to a considerable 
extent, since every motor 
must be protected by indi- 
vidual fuses (or circuit 
breakers). When there are 
a large number of these 
branch circuit cutouts re¬ 
quired, it is generally cus¬ 
tomary to group them to¬ 
gether as far as possible for 
convenience in renewal of 
fuses, and control of the 
circuits. This requires the 
use of panel boards which 
contain the essential fuses 
and control switches when 
these are essential. The 

panels are enclosed in metal cabinets and are set flush with or 
on the surface of the wall. Switchboards are not enclosed in 
cabinets. They contain the various switches and fuses which 
control the feeders for an installation. In general, they are 

285 



Fig. 209.—Lighting Panel Board. 
Ten circuit. 

Three-wire mains and two-wire branches. 
Switches and fuse clips in both branches 
and mains. Fuses not shown. Frequently 
switch and fuses in mains are omitted. 












































































-J 


286 PANEL BOARDS AND SWITCHBOARDS [CHAP. 17 

larger than the panel boards, and no branch circuits are sup¬ 
plied directly from them. 

292 . Cutout Cabinets. For a small number of lighting cir¬ 
cuits, individual cutouts enclosed in cabinets are used to pro¬ 
tect the branch circuits. They are usually installed in cast- 
iron or sheet-metal boxes. 

Lighting Panel Boards 

i , 

293 . Construction. Regular panel boards, such as are gen¬ 
erally used for branch lighting circuits, have all fuse clips and 
connections mounted on a single panel and connected to a set 



Fig. 210. —Sectional View of Lighting Panel Board and Cabinet. 

Flush-mounted Type. 


of copper busbars (Fig. 209). The branch circuit fuses may be 
either the Edison plug type or the cartridge type, as shown. 
Open-link fuses are still allowed by the Code, but they are seldom 
used in new work, and are not recommended, because of the 
damage which occurs to the panel board if fuses blow frequently. 
There is also the possibility of communicating fire if the door is 
left open. If the branch circuits are provided with switches, 
these may be of the knife-blade type, as shown in Fig. 209, or 
snap switches or push-button switches may be used. Knife 
switches should have a 30-ampere rating, to be substantial 
mechanically, although usually not more than 6 amperes is 
carried by the switch. For snap or push switches, the 10- 
ampere size would be used. Switches are not necessary on the 
branches unless the lights are controlled at the panel board. 
Knife switches are more satisfactory mechanically, but there is 
more danger of the operator receiving a shock than with the 
other styles of switches. The circuits should not be controlled 









































par. 294] 


LIGHTING PANEL BOARDS 


287 


% 

at the panel board where the operation is in the hands of un¬ 
skilled persons or the general public, because of the danger from 
shock and from tampering with the circuits. The switches are 
usually located between the busbars and the fuses (Fig. 210), 
In many cases, it is desired to group the control switches, but at 
the same time not allow access to the panel board. An excel¬ 
lent arrangement of this kind consists in mounting push switches 
in the cabinet and flush with the cover, so that the switches may 
be operated without opening the door of the cabinet. This is 
a good arrangement for stores. Each branch circuit should be 
numbered and a list of the circuits and the location of the lamps 
they supply should be mounted on the inside of the panel board 
door. The size of the busbars is determined by allowing 660 
watts per branch circuit (or 1320 watts if double-size circuits are 
used). If other panel boards are supplied from the same bus¬ 
bars the load on these panels must be included. Main switches 
and fuses (Fig. 209) are not generally necessary, but are con¬ 
venient for the purpose of cutting off all the circuits, so that 
repairs can be made without disturbing any other panel boards 
w r hich may be connected to the same feeder. In cases where 
two or three panels are carried on the same feeder, it is common 
to supply the largest panel directly from the feeder, and to run 
mains from cutouts located on this panel, to the other panel 
boards. When this is done, these circuits should be connected 
back of the main switch and fuses. The panels are generally 
made of slate, although marble is sometimes used. The slate 
should be at least | in. thick for panels having about fifteen 
circuits or less; larger panels should be If in. thick. 

294. Cabinets and Trim. Panel boards are enclosed in sheet- 
steel boxes or cabinets which are open at the front and are large 
enough to contain the panels and leave room at the sides for the 
wiring (Figs. 210 and 211). When properly designed, it should 
be possible to install the panel after the cabinet has been erected. 
Cabinets are made for mounting on the surface of the wall or 
for flush mounting (Fig. 210). Cabinets with gutters are usually 
about 4 in. deep. Gutters are generally provided in cabinets 
for running the branch-circuit wires (Figs. 210 and 211). This 
space should be at least 3 in. wide and should extend around all 
four sides of the panel. Sometimes the wiring space is provided 
behind the panel. A gutter or equivalent wiring space is 
required when there are more than four branch circuits, unless 


288 


PANEL BOARDS AND SWITCHBOARDS [CHAP. 17 


each circuit leaves the cabinet directly in line with its branch 
cutout. Usually this cannot be clone,’since most of the circuits 
enter either at the top or bottom and must be carried around 
the sides of the panel (Fig. 211). The gutter is separated from 
the panel by a slate frame which is made easily removable to 


facilitate connecting the circuits 



These frames should be at 
least \ in. thick. The cabi¬ 
nets are closed by a matt or 
trim which contains the door. 
This door is the same size 
as the panel, so that when 
open the gutter is not ex¬ 
posed. For flush mounting 
the trim projects a slight 
distance beyond the edges of 
the cabinet to cover any 
small break in the plaster 
(Fig. 210). Both steel and 
wood trims are used. For 
industrial plants, etc., steel 
trim would generally be used. 
For office buildings, stores, 
etc., wood trim to match the 
finish is more common. 

295. Types and Sizes o: 
Panel Boards. Standard 
lighting panel boards are 
made for two-wire, 125- or 
250-volt branches, with 
mains, or busbars arranged 


Fig. 211. —Lighting Cabinet, with 
Trim Removed. 

Showing method “of carrying wares 
through gutter from branches to conduit. 


for two-wire, three-wire or 
three-phase supply. As a 
rule, the double-branch type 
(Fig. 209) is most satisfac- 
' tory, because it gives a bet¬ 
ter proportion for the door, but for a small number of circuits 
where only a narrow panel could be used, single-branch 
panels, having branches only on one side of the busbars, may 
be used. The spacing of the parts on a three-phase panel is 
different from a three-wire board, so that care should be 
taken in ordering. Standard panel boards are made with 













par. 296] 


POWER PANEL BOARDS 


289 


from two to thirty-two circuits. When a greater number of 
circuits must be controlled from one point it is best to use 
double panels with two doors. The dimensions of panels and 
cabinets vary somewhat with different manufacturers, but the 
dimensions given in Table 37 are representative of good 
practice. 

296. Power Panel Boards. When there are a number of 
motors which can be conveniently supplied from one distributing 
point, a panel board may be used to supply the branches. The 
arrangement is similar 
to lighting panels except 
that the branch circuits 
are usually larger than 
30 amperes (Fig. 212). 

These panels must be 
made to order because 
of the range in capacity 
required. The branch 
circuits must be pro- • 
vided with fuses. Knife 
switches are also de¬ 
sirable so that the motor 
control equipment and 
branch wiring can easily 
be cut off for inspection 
or repairs. Knife 
switches and fuses in the main busbars are not necessary unless 
other panels are supplied from the same feeder. In some cases, 
circuit breakers are mounted on panel boards. The cabinets 
used for power panel boards are of the same general design as 
for lighting panels, except they are usually larger. If circuit 
breakers are used, an ample space must be left between the 
breaker when open and the cabinet or the door. 

297. Switchboards. We are concerned here only with switch¬ 
boards for the control of lighting or power feeders. Where 
the installation is small, a service switch and cutouts are all 
that is required (Fig. 229). For large installations, whether 
supplied from an isolated plant or a central station, there will be 
required a number of feeders, which must be brought together 
at the point of supply and separately controlled. In large 
office buildings, etc., the switchboards required for these feeders 



Fig. 212. —Power Panel Board. 
For a 115-volt, two-wire, d.c. system. 








290 


PANEL BOARDS AND SWITCHBOARDS [CHAP. 17 


reach large proportions. The panels used for switchboards 
are commonly slate or marble and are 2 in. thick, except for 
small installations. They are supported on a pipe or angle iron 
framework. Back-connected switches and circuit breakers are 
used so that all connections are made in the rear of the board. 
For lighting feeders the usual arrangement is to provide a knife 
switch and fuses for each feeder. The fuses should be mounted 



Fig. 213.—Typical D.C. Switchboard. 

From left to right the arrangement is: Two generator panels, one power 
feeder panel with circuit breakers for three feeders, one lighting feeder panel 
with fused knife switches for four feeders. (General Electric Co.). 


preferably on the front of the board if there is room. For power 
feeders, a switch and fuses are often used, but, in general, a 
circuit breaker is better for the reasons given in paragraph 274. 
If an oil circuit breaker is used, the switch is omitted, since the 
breaker takes its place. It is also possible to obtain circuit 
breakers which will trip out if closed on an overload, and in such 
cases the switch may be omitted. Oil circuit breakers are, in 














par. 297 ] 


SWITCHBOARDS 


291 



general, better for use ona.c. power circuits than carbon breakers, 
because the latter require considerable space between adjacent 
breakers to avoid danger from a flash at one breaker spreading 
to another. For economy of space, it is generally necessary to 
placfe two or sometimes three breakers on the same panel one 
above another. W ith carbon breakers, there is great danger of 
the flash from one breaker spreading to those above and causing 


Fig. 214.—Typical A.C. Switchboard. 

From left to right the arrangement is: Two generator panels with non-auto¬ 
matic oil switches, one three-phase feeder panel with automatic oil circuit 
breakers for two power feeders, one single-phase feeder panel with automatic 
oil circuit breaker for a lighting feeder. (General Electric Co.). 

a short-circuit. Typical switchboards for small isolated plants 
are shown in Figs. 213 and 214. It will be noted that enclosed 
fuses and carbon circuit breakers are used for the d.c. board and 
oil circuit breakers for the a.c. board. The same general plan 
is followed for large installations where, however, more instru¬ 
ments are generally used. For a system using central station 
service, the switchboard shown in Fig. 228 is representative. 
Fuses or circuit breakers should be adjusted for the maximum 
current allowed for the particular feeder, in accordance with 












292 


PANEL BOARDS AND SWITCHBOARDS [CHAP. 17 


Table 36, regardless of the fact that the actual load may be con¬ 
siderably less. This reduces the probability of the circuit 
being opened. Meters are sometimes used on feeder switch¬ 
boards. There is not much need for ammeters on lighting 
feeders, but for power feeders they are useful and are fre¬ 
quently employed. A voltmeter equipped to read grounds on 
the system is a convenience. It is usually desirable to provide 
watt-hour meters for the lighting and power loads and in some 
cases for individual feeders. Only by this means can a proper 
record of the operation of the system be kept. 



CHAPTER 18 


ARRANGEMENT OF CIRCUITS 

298. Parts of a Circuit. The connection between the point 
of supply and the individual lamps or motors is made by means 
of feeders, mains and branches. The feeder (Fig. 215) is the 
part of the circuit between the switchboard and the first dis- 


Panel Box Panel Box 



Fuses and switches for branch circuits and mains not shown. Usually 
motors are run on separate feeders and not on a lighting feeder as shown. 

tributing centre (A). The feeder is usually the longest part of 
the circuit. The mains (sometimes called sub-feeders) connect 
the first distributing point (A) with the other points ( B , C, D, 
etc.). Sometimes there are no mains. The branches supply 
individual motors or groups of lamps. In general, as the mains 
are smaller than the feeder, fuses are required at the junction 

293 


























































294 


ARRANGEMENT OF CIRCUITS 


[chap. 18 


point A to protect these mains. The branches must also be 
fused to protect the lamps or motors. When the electricity is 
supplied by a central station, a service or service mains are 
run in from the street circuits to the switchboard in the build¬ 
ing. This service and the meter for recording the amount of 
electricity consumed are installed by the central station com¬ 
pany, usually without charge to the customer, except where 
there are special conditions such as a long service or where spe¬ 
cial transforming devices are required.* 

299. Main and Feeder Systems. The simplest arrangement 


of lighting circuits is shown i 

n Fig. 216. The mains are run the 

L, 

0 0 0 0 0 0 a 

00000,000 

J T 1 

Maui 

^ / \ 

Main Lamps 

4 * > 

0 0 0 0 0 0 \b 

0 0 0 0 0*'0 0 0 

.11 1 

I 

" J 

0 P 0 0 0 0 Jo 

]00000000 

1 

Fig. 216.—Lighting System 

length of the room and the ] 
fuses in the rosette used wit 
tected by fuses in the cutouts 
at one time quite common for 
Code for such places. The 
the rosettes are likely to gn 
quently in such locations thal 
Also, groups of lamps cannot 
desirable. Furthermore the 
lower voltage on the lights a 

7 

Feeder 

. Using mains and fused rosettes. 

amps are supplied directly through 
h each lamp. The mains are pro- 
, a, b and c. This arrangement was 
mill work and is still allowed by the 
disadvantages are that the fuses in 
re trouble, and the rosettes are fre- 
t it is difficult to replace blown fuses, 
be controlled separately, as is often 
re is likely to be a considerably 
t the ends of the mains. A better 


arrangement (generally employed at present) is to install the 
lamps without individual fuses, in groups taking not more than 

* See paragraph 305. 


























pah. 299 ] 


FEEDER SYSTEMS 


295 


660 watts (or in special cases, 1320 watts*). The branch cir¬ 
cuit supplying this group is then run back to a distributing cen¬ 
tre or panel board, where fuses are provided (Fig. 217). This 
panel board would be located in an accessible place to facilitate 
replacement of the fuses. This arrangement gives greater uni- 


Pancl Board 




2 9 0 0 0 0~5 


w - 

T-Jr 





,0 0 0 0 0 0 

Q 

43 

U 

a 





J 

Branches 

C3 

U 

w 

-~lr 



L=-1 oooooo 


Feeder or Main 


Fig. 217.—Lighting System. 

With panel board and branch circuits. 


formity of voltage at the lamps and makes their control more 
flexible. For power circuits, feeders are frequently run the 
entire length of a building and branch circuits for individual 
motors tapped off at different places (Fig. 218). When there 
are several motors close together, a better arrangement is to use 



Fig. 218.—Power System. 

Showing method of tapping directly from feeder. Starting devices not shown. 


panel boards similar to those used for lighting, except that they 
are of larger capacity. The advantage of this method is that 
the cutouts for the branch circuits can be placed in a readily 
accessible position, where they can be properly enclosed and 

* See paragraph 303. 



































































296 


ARRANGEMENT OF CIRCUITS 


[CHAP. 18 


protected. With the arrangement shown in Fig. 218, the 
cutouts would be scattered over the building and would fre¬ 
quently be located in inaccessible places. As a rule, more than 
one panel board is connected to a single feeder, for the sake of 
economy in the installation. The method of feeding the panel 
boards will depend upon the size of the building and the charac- 




Fig. 219. —Feeder System for Fig. 220. —Feeder System for 
Small Buildings. Small Building. 

Showing use of a single feeder. 

ter of the service supplied. For small office buildings and indus¬ 
trial establishments where only a few floors are to be supplied, 
the arrangement shown in Fig. 219 is used. This method has 
the disadvantage that the voltage of the lamps on the upper 
floors is lower than on the others because the drop cannot be 
equalized. Also it is not possible to control a portion of the 
lights separately from the switchboard. The mains connecting 
the panels are usually made smaller than the feeders (Fig. 219), 

































































par. 299 ] 


FEEDER SYSTEMS 


297 


and, therefore, fuses would be required at the point where each 
main starts. Panel board D, for example, might have several 
sets of fuses between it and the switchboard, and the chance of 
trouble is thereby made greater. A better arrangement is 
shown in Fig. 220. It would be possible, by this method, to 
get the same drop on each feeder, so that the voltage at all the 
panel boards would 
be practically the 
same. Besides 
this, it is possible 
to control a portion 
of the lamps or 
motors independ¬ 
ently of the others. 

Usually it is most 
economical to sup¬ 
ply not more than 
three panel boards 
from one feeder, 
but no exact rule 
can be given. The 
arrangement for 
an office or loft 
building or similar 
service is shown 
in Fig. 221. The 
best arrangement 
is that in Fig. 221a, 
because it gives 
more uniform volt¬ 
age. In large 
buildings, the floor 
area is so great 
that all the lamps 



Fig. 221.—Feeder System for Office or Loft 

Building. 

Centre feed. b. Bottom feed. 


a. 


or motors on a floor cannot be supplied from a single panel board 
without excessive drop in the branches. Where a number of 
panel boards are located on the same floor, it is best, if possible, 
to feed them vertically in groups as shown in Fig. 222. It 
sometimes happens that it is necessary to carry an entire floor 
on one feeder so that the power can be metered and controlled 
independently. For such requirements, the method shown in 

































































































298 


ARRANGEMENT OF CIRCUITS [CHAP. 18 


Panels 


Panels 


Panels 



Fig. 223 is satisfactory and would generally be employed for the 
first floor of an office building or store and for each of the floors 

below ground. 
For the upper 
floors a vertical 
arrangement sim¬ 
ilar to Fig. 222 is 
preferable and is 
generally used. 
Motor feeders and 
panel boards are 
generally entirely 
separate from the 
lighting circuits, 
but the same gen¬ 
eral rules apply. 

300. Separate 
Control of Special 
Groups of Lamps. 
In large office and 
loft buildings it is 
desirable to control 
the hall lamps 
separately from 
the rest of the 
lighting load. 
These lamps are, 
therefore, supplied 
from independent 

feeders, thereby making it possible to cut off the supply from 




Rising Point A 


Rising Point B 


Rising Point C 


Fig. 222. —Feeder System for a Large 
Building. 



Fig. 223.—Method of Feeding Panel Boards. 

the rooms when they are not occupied. In hotels, this arrange¬ 
ment is not necessary, as the feeders are left on continually. 



































































































PAR. 301] CONTROL OF BRANCH CIRCUITS 


299 


In some cases the hall circuits are divided between two feeders, 
so that part of the lamps can be extinguished at night after 
the tenants have left the building. This arrangement is quite 
expensive and is used only in large buildings. 

301. Control of Branch Circuits. For lighting service the 
branch circuits are sometimes controlled by switches in the 
panel boards. Knife switches are often used in such cases, but 
there is always danger of the operator receiving a shock. This 
arrangement is, therefore, suitable only when the control of the 
lamps is in skilled hands. Usually it is best to control the lamps 
by switches separate from the panel. These switches may be 
built into the panel board in such a way that they can be oper¬ 
ated without exposing the panel board and fuses,* or they may 
be located at convenient places near the lamps. For office 
buildings and similar places, switches controlling the lamps 
should be located near the entrance to the room and on the lock 
side of the door. In large rooms, several switches may be used 
so that only the lamps required need be used at any time. In 
factories or stores, the switches may be located on columns or 
side walls and should be grouped together as much as possible. 
The switches should be about 4 ft. above the floor. Push¬ 
button switches are used for the best installations and rotary 
snap switches for cheaper equipments. Single-pole switches 
are allowed by the Code for branch circuits carrying not 
more than 660 watts, except in damp places, where double¬ 
pole switches must always be used (Rule 24c). For services, 
double-pole switches are required, and they are better 
for all work because they open both sides of the circuit. 
This divides the arc and causes less burning of the con¬ 
tacts. Double-pole switches are used generally for the better 
grade of work, especially in industrial plants, where the danger 
of grounds on the branch circuits is greater than for residences, 
etc. Three-way and four-way switchest are considered as 
single-pole switches. Nothing smaller than a 10-ampere switch 
should be used, because the 5-ampere size is not as sub¬ 
stantial mechanically. In rooms open to the public and in 
public halls, switches operated by a key are frequently used. 
As a rule, lamps are seldom controlled individually by means of 
key sockets. Usually the fixtures, to be most effective, must be 
located so high that the sockets cannot be reached. Even 
* See paragraph 293. . - t Paragraph 267. 


300 


ARRANGEMENT OF CIRCUITS 


[CHAP. 18 


where the sockets are within reach, it is best to provide switches 
to encourage the shutting off of the lamps when not needed. All 
motors must be provided with some form of starting device or 
controller which must be in sight of the motor in an accessible 
position (Rule 8c). A switch disconnecting all the wires of the 
branch circuit must also be provided, unless the starter is de¬ 
signed to open all wires. This switch must be in sight of the 



Fig. 224. —Arrangement of Control Devices for a Motor. 

motor. Each wire of the circuit must be fused. Fig. 224 
shows an arrangement for a d.c. motor. 

302. Special Circuits. For hall lights, it is convenient to con¬ 
trol the lamps from two or more points. This can be done by 
using three-way switches (Fig. 225). It will be seen that the 
lamps may be lighted or extinguished from either switch, regard¬ 
less of the position of the other. For control from three points, 
one four-way switch and two three-way switches are required. 
For each additional point of control, another four-way switch 
must be used, connected in the same manner as the four-way 
switch in the diagram. Large fixtures or “ electroliers ” are 





PAR. 302 ] CONTROL OF BRANCH CIRCUITS 


301 


frequently wired so that the lamps can be controlled inde¬ 
pendently in two or three groups.* Electrolier switches are used 
for this purpose (Fig. 225). The combinations shown are two 
only of several different arrangements which can be secured by 
selecting the proper style of switch. For loft buildings, it is 
sometimes desired to light the hall lamps on only one or two floors 
at a time. This may be accomplished by the arrangement 
shown in Fig. 226. In residences, a master switch is sometimes 



Fig. 225.—Diagrams for Special Lighting Circuits. 

a. Use of 3-v r ay switches for controlling lamps from two points, b. Same 
as (a) with different method of connection, c. Use of 3- and 4-way switches 
to control lamps from three places. For each additional place, a 4-way switch, 
connected the same as the middle switch must be used. d. Two-circuit elec¬ 
trolier switch. Arrangement gives: 1st, off; 2d, 1 on; 3d, 1 and 2 on; 4th, 
2 on. e. Three-circuit electrolier switch. Arrangement gives: 1st, off; 2d, 
1 on; 3d, 1 and 2 on; 4th, 1, 2 and 3 on. Other styles of electrolier switches 
can be obtained to give different combinations. 


provided to control all or a part of the lamps in the house from 
one point (for example, the owner’s bedroom), regardless of the 
position of the individual control switches. The arrangement 
shown in Figs. 227a and b is suitable only for a small number 
of lamps, since the master switch cannot be used to control more 
than 660 watts. One method of overcoming this difficulty is 
to use a special form of push-button switch.* The arrange¬ 
ment of connections is shown in Fig. 227c. The special push¬ 
button switch is used for the regular control of the lamps and 

♦Electrical World, March 11, 1910. 

































302 


ARRANGEMENT OF CIRCUITS 


[chap. 18 


each of these switches is connected to a common wire leading 
to the master switch. By this means a portion of the lamps in 

each large fixture can be controlled 
from one point. 

303. Arrangement of Branch Cir¬ 
cuits. Branch lighting circuits are 

almost always two wire. Such a 
circuit, which is protected by one 
set of fuses, cannot have more than 
16 medium or 25 candelabra size 
sockets or receptacles, and the load 
must not exceed 660 watts (Rule 
23d). It is better, however, to limit 
the number of sockets to 12. It 
will be seen that the Code allows 
the use of sixteen 40-watt lamps on 
one circuit, but as the lamps are 
usually larger than this, the number 
of sockets would generally be less. 
In counting the number of sockets, 
plug outlets must be included. 
Where the equivalent of No. 14 
rubber-covered wire can be run di¬ 
rectly into keyless sockets or recep¬ 
tacles 1320 watts and 32 sockets 
are allowed (Rule 23d). In arrang¬ 
ing the branch circuits, an allowance 
should always be made for future 
additional load. The amount of this 
allowance varies, but as a general 
rule, for factory lighting, the load 
should be about 90 per cent, and 
for office buildings or stores from 80 
to 90 per cent of the maximum load 
allowed on a branch. The larger 
allowance in the latter case is made 
to take care of the requirements of 
different tenants. It is, of course, 
important to have as much load 
as possible on a branch circuit (bearing in mind the above 
limitations), because each additional branch adds consider- 



Fig. 226. — Connections 
for Hall Circuits in 
Tall Buildings. 

A double-pole switch is pro¬ 
vided at first and top floors, 
others have three-way switches. 
Closing switch on first floor 
lights the lamp on this floor 
and the floor above. Oper¬ 
ating the switch on the second 
floor extinguishes the lamp 
on first floor and lights that 
on third floor. This is re¬ 
peated on each floor up to the 
top. The arrangement can be 
used for any number of floors. 
The switch on each floor must 
be operated in passing. 


































































PAR. 303] ARRANGEMENT OF BRANCH CIRCUITS 303 

ably to the cost of the installation. The lamps on a branch 
circuit should be grouped together as much as possible to 
avoid very long circuits/ Table 41 gives an indication of the 
limiting lengths of such circuits and will be found convenient 
when laying out branch wiring. For large public rooms, where 
it is important that the blowing of a feeder fuse shall not extin¬ 
guish all of the lights in the room, the branch circuits are sup¬ 
plied from two or more feeders. As a rule, it is desirable to use 
only one size of wire for all branch circuits in a given installation 
and to so locate the panel boards that none of these circuits will 
exceed a suitable length. It is well to control the lamps near¬ 
est the windows separately from the others so that they can 



Fig. 227.—Master Switch Circuits. 

a. With 3-way switches, b. With 3- and 4-way switches, c. With special 
push-button switches. 

be extinguished when artificial light is needed only in the darker 
parts of the room. When branch circuits are supplied from a 
three-wire main it is important to divide the branches evenly 
between the two sides of the system, so that each panel board 
will have an evenly balanced load. Otherwise the neutral wires 
between panel boards will carry a balancing current, which will 
cause a disturbance in the voltage of the lamps, even if the total 
load on the system is so well balanced that no neutral current 
is drawn from the service, d his means that three-wire panels 
and not two-wire should be used when three-wire mains and 
feeders are employed. For motor branches, a separate circuit 
is used for each machine, unless the motors are very small. 
Further details regarding motor branches are given in Chapters 

19 and 20. 































304 


ARRANGEMENT OF CIRCUITS [CHAP. 18 


304. Location and Size of Panel Boards. For office buildings, 

the panel boards are located in the halls to avoid disturbing the 
tenants when fuses are to be replaced. In stores and industrial 
establishments, where large spaces must be supplied, the panels 
are located at some accessible place in the room on a side wall or 
column. In every case the panel should be located as nearly as 
possible at the centre of the load it supplies so that the branch 
circuits will all be nearly the same length. For lighting panel 
boards the exact location and the number used will depend upon 
the amount of load and the floor area. In a building of any 
considerable size it is necessary to have more than one panel 
board on a floor to avoid excessively long branch circuits. Ref¬ 
erence to Table 41 shows that with No. 12 wire the load centre* 
of the branch can be about 110 feet away from the panel, so 
that the branch could feed lamps about 120 feet away in any 
direction. TJae distance between panels should, therefore, be 
not much over 250 ft. If No. 14 branch circuits are used the 
panels would have to be closer. As far as possible the panels 
on the different floors should be located in vertical lines to avoid 
offsets in the feeder circuits. It is much more difficult to con¬ 
ceal the large conduits required for feeders and mains in the 
floors of a building where horizontal runs are made than it is 
to conceal them in vertical runs, by means of wire shafts. Spare 
circuits should always be provided on the panels. In general 
there should be two spare circuits for a board having less than 10 
circuits; four for 10 to 20 circuits and from six to eight for more 
than 20 circuits. For motor panels the same rules regarding 
location, etc., apply. The allowance of spare circuits must be 
made after a careful consideration of the possibility of additional 
motors being required. This would vary widely in different 
cases. 

305. Location of Switchboards and Service Connections. If 

the building is supplied from a private plant, the feeder switch¬ 
board is naturally located in the engine room next to the gen¬ 
erator panels. If the supply is from an outside service, there is 
usually a choice of locations for the switchboard. It should not 
be placed at one end of the building but, instead, should be near 
the centre, so as to make the lengths of the various feeders as 
nearly as possible the same. If this is done, the voltages at 
different points of the system can be made more nearly equal, 

* See paragraph 318. 


par. 305 ] 


LOCATION OF SWITCHBOARDS 


305 



and there will be a considerable saving in the cost of the feeder 
system. Sometimes, when the private plant is a considerable 
distance from the building to be served, the electricity is trans¬ 
mitted in bulk, by a few large feeders, to a suitable distributing 
point in the building where the feeder switchboard can be 
located. Fig. 228 shows a typical switchboard where central 
station service is used. For small installations, there would be 


Fig. 228. —Feeder Switchboard. 

The large switches at the bottom are tie switches. (Krantz Mfg. Co.). 


at the service point only the service switch and cutout required 
by the Code, together with the watt-hour meter for recording 
the total power consumed. From this point, a single feeder (or 
possibly one for lights and another for motors) would run to the 
first panel board. Fig. 229 shows the arrangement of a service 
for a residence. The location chosen for the service switchboard 
should be clean and as dry as possible, and provision should be 










306 


ARRANGEMENT OF CIRCUITS 


[chap. 18 


made to properly enclose the board to protect it from inter¬ 
ference. There should be, at least, 3 ft. clear space between 
the ceiling and the top of the board. At least 18 in. free space 
must be provided between the wall and the apparatus on the 
back of the board, and it is better to allow more space than this, 

particularly if the fuses are 
on the back. To prevent 
the accumulation of rub¬ 
bish back of the board, the 
panels should not extend 
down to the floor. 

306. Arrangement of 
Feeders and Mains. With 
the location of panel boards 
and switchboards settled, 
the feeder system can be 
planned. The considera¬ 
tions given in paragraph 
299 will serve as a guide 
in doing this. In planning 
a feeder layout it is cus¬ 
tomary to sketch out the 
feeder system according to 
several arrangements and 
then to compare these with 
regard to first cost and 
convenience of arrange¬ 
ment. To do this, the 
panel boards are located on 
a riser diagram, which indicates the various floor levels and 
shows the approximate location of the panel boards. The 
feeder system is then sketched in and the loads calculated. 
Fig. 231 shows a riser diagram for a factory building. 

307. Limitations in Size of Circuits. The methods of deter¬ 
mining the sizes of the circuits are given in Chapters 19 and 20. 
There are, however, certain limitations in the size of these cir¬ 
cuits which will have a bearing upon the arrangement used. As a 
rule, it is seldom desirable to use a cable larger than 1,000,000 cir. 
mils, especially for a.c. work at 60 cycles.* Where the circuits 
are to be run in conduit and all wires of a circuit must be placed 

* See paragraph 323. 



Fig. 229.—Service for a Residence. 













































par. 308 ] 


GROUNDING CIRCUITS 


307 


together, 500,000 cir. mils is about the limit in size because of 
the large conduits required. When horizontal runs are to be 
concealed in floors, a 2|-in. conduit is about as large as can gen¬ 
erally be used. This limits the size of conductors to 300,000 
cir. mils where two or three wires are installed together. Ver¬ 
tical runs are concealed in wire shafts so that larger sizes may be 
used in such cases. In open wiring, there is not as much lim¬ 
itation in size, but usually it is desirable to so subdivide the load 
that the individual feeders need not be larger than 1,000,000 
cir. mils. Generally they are considerably smaller.* 

308. Grounding Circuits. Low-voltage circuits (i.e., 550 
volts or less), for lighting or power service, must be grounded if 
supplied from a central station or a transformer having a pri¬ 
mary voltage greater than 550 volts (Rule 15). For three-wire 


i 




I-OOOQ 


,V • v v v v 


no' 

Volts 


Fig. 230.—Grounding Low-voltage Circuits. 

a. Not grounded, b. Grounded. 


systems, either direct or alternating current, the neutral is 
grounded. For systems supplied from transformers having no 
neutral one side of the circuit must be grounded if the voltage 
between ground and any other wire is not over 150 volts. For 
higher voltages grounding is optional.' The purpose of ground¬ 
ing is to prevent any chance of an excessive voltage occurring 
on a low-voltage system. With circuits fed from transformers 
there is always a chance that the insulation between the sec¬ 
ondary and primary of the transformer may be weakened by 
lightning or other cause and a leakage occur (Fig. 230). If the 
low-voltage system is not grounded, a leak from the primary side 
would cause an excessive voltage between the wiring and ground, 
even if the insulation of the primary feeder system were perfect. 
If this occurred, a person touching a metal socket or a switch, 
especially if standing on a damp floor or in contact with water 

* Other reasons for reducing the size of a.c. circuits are given in paragraph 
325. 



















308 


ARRANGEMENT OF CIRCUITS 


[chap. 18 


piping (as in a bath room), would receive a severe and possibly 
a fatal shock, and fires would probably be started by arcs 
caused by the breaking down of the insulation. If the circuit 
is grounded, a leak in the transformer insulation would cause a 
current to flow to ground, the fuses would blow and thus dis¬ 
connect the circuit. In this case, a person touching the fixtures 
would be protected. The frames of motors and generators 
must be grounded wherever feasible (Rule 8a). Where this is 
impracticable the motor may (by special permission) be placed 
on wooden base frames or a wooden floor which is clean and free 
from moisture. 




V 


* 


CHAPTER 19 


CALCULATION OF D.C. SYSTEMS 

309. Factors which Affect the Size of a Circuit. All con¬ 
ductors used for electric wiring must be large enough to carry 
the current without overheating, and without an excessive loss 
of voltage and must also be strong enough mechanically to with¬ 
stand any strains\to which they may be subjected. The Code 
specifies definitely the largest current which a wire should carry,* 
but makes no requirements regarding voltage loss or drop. For 
mechanical reasons, no wire smaller than No. 14 B. & S. gauge is 
allowed, except in fixture wiring, where No, 18 is permitted. 

Calculation of Load on a Circuit 

310. Two-wire Branch Circuits. For lamps or heaters, the 

total current is found by dividing the total watts load on the 
branch circuit by the voltage. Where plug outlets for portable 
lamps are included these should be figured at not less than 40 
watts each, and more if it is definitely known that they will be 
used for heavier loads. If small motors are also attached to 
the lighting circuit, the full-load current of these should be 
included. When the current taken by each outlet is known, the 
total current can be found by adding these currents together. 
Large motors (above one-fourth horsepower) must be supplied 
by individual branch circuits. The full-load current can be 
obtained from Table 21. 

311. Determining Load on Lighting Panel Boards. For a 
two-wire system, the actual load on the panel is the sum of the 
currents required for the branch circuits as calculated in par¬ 
agraph 310. The maximum load should also be estimated by 
allowing 600 watts per branch (or 1200 watts if double size cir¬ 
cuits are used), including the spare circuits. For a three-wire 
system, the current for one of the outside terminals of the panel 
board is found by dividing the watts load connected to this tcr- 

* Table 36. 

309 


310 


CALCULATION OF D.C. SYSTEMS [CHAP. 19 


minal by the voltage between terminal and neutral. The cur¬ 
rent should be calculated for the side of the system having the 
greatest connected load, although in a properly designed system 
there should be very little difference between the two sides. 


PanelNo. 6 



Fig. 231. — Riser Diagram for a Factory. 


The actual loads and maximum loads should be figured as in a 
two-wire system. 


Example 1. One floor of a factory is lighted by 108 units each 
rated at 100 watts. The units are arranged in 6 rows of 18 each 
giving two branch circuits per row or a total of 12 branches. (Double 
size circuits are used.) There are also three circuits for portables. 
An 18-branch panel board should be used. For a two-wire, 120-volt 
system we have: 

l 

Lamp load 108 X100 = 10,800 watts 
Portables 9 X3 X40 = 1,080 


Total 11,880 

The actual current is therefore 11,880-r-120 =99 amperes. 
The maximum load is: 

Lamp circuits 12 X1200 = 14,400 watts 
Portable circuits 3X600= 1,800 

Spare circuits 3 X600 = 1,800 


18,000 


Total 

The maximum current is 18,000 -^-120 =150 


amperes. 
























PAR. 312 ] 


CALCULATION OF LOAD 


311 


Example 2. If a 240-120-volt, three-wire system is used, we have: 
Lamp load 54 XI00 =5400 watts 
Portables 9 X2 X40 = 720 


Total for one side 6120 

The actual current in the outside wire is 6120=120 =51 amperes. 
The maximum load is: 

Lamp circuits G X1200 =7200 watts 
Portable circuits 2X600 = 1200 
Spare circuit 1 X600 = 600 


Total for one side 9000 
The maximum current is 9000 =120 =75 amperes. 

The current in the neutral is practically zero and need not be calculated. 


Where there are a number of panel boards, the loads should be 
tabulated as in paragraph 312. 

312. Determining I?oad on Lighting Feeders or Mains. After 
the arrangement of feeders and mains has been made in accord¬ 
ance with one of the schemes described in Chapter 18, the load 
on each part of the circuit can be determined. This is com¬ 
puted from the loads on the panel boards which are supplied by 
the circuit. Both the actual load and the maximum load should 
be calculated. A riser diagram* is of assistance in determining 
these loads, as it can be seen at a glance just what panels are 
supplied by each circuit. 

Example. Fig. 231 gives a riser diagram for the lighting feeders of 
a factory building and the tabulation below gives the loads on the 


panels. The feeder loads are 

as follows: 


Feeder A: 


Actual. 

Maximum. 

Panel No. 

1 

18 amperes 

30 amperes 


2 

99 

150 


3 

99 

150 

Total 


216 

330 

Feeder B: 




Panel No. 

4 

30 amperes 

50 amperes 


5 

90 

140 

% 

6 

90 

140 

Total 


210 

330 


* See paragraph 306 






312 


CALCULATION OF D.C. SYSTEMS [CHAP. 19 


Loads on Panel Boards 


For a two-wire, 120-volt system 


Floor. 

1 

Panel 

No. 

2 

Circuits 
in Use. 

3 

Total 

Circuits 

Pro¬ 

vided. 

4 

Load 

in 

Watts. 

5 

Actual 

Load, 

Amperes. 

6 

Maxi¬ 

mum 

Load, 

Amperes. 

Basement. . . 

1 

4 

6 

2,160 

18 

30 

First floor. . . 

2 

15 

18 

11,880 

99 

150 

Second floor.. 

3 

15 

18 

11,880 

99 

150 

Third floor. . 

4 

8 

10 

3,600 

30 

50 

Fourth floor. 

5 

12 

16 

10,800 

90 

140 

Fifth floor. . . 

6 

12 

16 

10,800 

90 

140 


A three-wire system would be calculated in a similar manner. 
For the example chosen, Feeder A would carry a maximum load 
of 165 amperes and an actual load of 111 amperes. For Feeder 
B the values are 165 amperes and 105 amperes. 

313. Determining Load on Power Panel Boards. Since 
d.c. power circuits are almost always two-wire, only this system 
will be considered. The full-load current of each motor can be 
determined from the name plate on the machine or can be 
closely approximated from Table 21. In calculating the load 
on a power panel board, the maximum running load must be 
estimated as nearly as possible. Under usual conditions, the 
motors would not all carry full load at the same time, so that the 
maximum load would be less than the total full-load capacity of 
the motors connected to the panel. In estimating this maximum 
load, it is usual to employ a demand factor which is the ratio of 
the maximum load to the total full-load rating of the motors 
connected to the panel. This demand factor varies from about 
0.40 to 1.25, depending upon the nature of the work and the 
number and size of the motors (see Table 38). The factor is 
larger where only a few motors are connected to the feeder. 
The connected load can be found by adding together the full¬ 
load currents of all the motors, including a proper allowance for 
any spare capacity in the panel board. Multiplying this con¬ 
nected load by the demand factor would give the maximum load 
on the panel board. This would be the maximum running cur¬ 
rent required for the entire panel board. Where one of the 


















PAR. 314 ] 


CALCULATION OF LOAD 


313 


motors is Very much larger than the others, the starting condi¬ 
tions should also be considered. To do this, the maximum load 
on the panel when all motors but the largest are running should 
be found in the manner just described. To this should be added 
the starting current of the large motor. For shunt or compound 
motors, the starting current may be assumed to be 1.25 times 
the full-load current unless more definite information is avail¬ 
able. For series motors, the starting current would be at least 
1.50 times full-load current and might be greater than this for 
some types of service (see paragraph 315). 

Example 1 . Shunt motors, 120-volt system. 

Running conditions: 

1-5 hp. motor 40 amperes 

1-3 24 

1 - 2.5 20 

2- 1 17.6 

1-3 (spare) 24. 


Total 125.6 amperes 

From Table 38, the demand factor is found to be about 0.65. 

Hence the maximum running current is: 

125.6X0.65 =81.7 amperes. 

Example 2. If the panel which supplied the motors in Example 1 
also had connected to it a 50-hp. motor, we would have to consider 
the starting conditions as follows: 

Running load, six motors 81.7 amperes 

Starting load 50-hp. motor, 356X1.25 =443 


Total 524.7 amperes 

314. Determining Load on Power Feeders and Mains. A 

riser diagram of the power feeder system should be prepared to 
assist in determining the load. This diagram is similar to the 
lighting riser diagram already described. Where the motors 
are all of about the same size, the load on the feeder can be 
found by adding the maximum running loads of the panel boards. 
If, however, there is one very large motor, the load produced 
when this is started and all the others are running should be 
determined in the manner described in the previous paragraph. 
Where there are several panels supplied by one feeder, the load 
would actually be less than the sum of the loads of the different 
panels, because their maximums would not all occur at the same 




314 


CALCULATION OF D.C. SYSTEMS [CHAP. 19 


time. Adding these loads, therefore, gives some spare capacity, 
which is generally desirable. It is always well, however, to 
calculate the maximum running load on the feeder by adding 
the full-load currents of all motors on the feeder and multiplying 
by the proper demand factor as found from Table 38. To this 
must be added a proper allowance for future load, which may be 
25 per cent or more. The amount of this allowance depends 
a great deal upon whether the feeder is in a factory, where the 
load requirements may increase considerably due to the addition 
of more or larger machines, or in an office building where there 
is little chance for an increase in load. 

Example 1. For a 230-volt system a feeder has the following 
connected load. All are for group drives: 


1-25 hp. motor, 

91 amperes 

3-20 

219 

4-10 

152 

6- 5 

120 

Total 

582; amperes 


From Table 38, the demand factor is found to be about 0.70. 

Hence the maximum running current would be: 

582X0.70=407 amperes. 

Allowing 15 per cent for future additions, the current would be: 

407X1.15=469 amperes. 

315. Determining Size of Branch Circuits and Fusing. The 

smallest branch circuit allowed is No. 14, but for long runs, espe¬ 
cially for motors, it is better practice to use No. 12. For lighting 
circuits, the branch loads are limited to 660 watts, or in special 
cases 1320 watts.* The circuits may be fused as follows: 


Fuses for Branch Lighting Circuits 


Voltage. 

Size of Fuse. 

660-watt Branch, 
Amperes. 

1320-watt Branch, 
Amperes. 

125 volts or less. 

126 to 250 volts. 

10 

5 

20 

10 



* See paragraph 303. 















PAR. 316 ] DETERMINING SIZE OF A CIRCUIT 315 

Each arc lamp circuit must have a carrying capacity and be 
fused for a current at least 1.50 times the rated current of the 
lamp to allow for the starting conditions. Branch circuits for 
motors must have a carrying capacity at least 25 per cent 
greater than the full-load rated current (Rule 86). The size 
of wire is determined from column A or B of Table 36, depending 
upon the kind of wire used. When branch circuits are fused for 
a capacity 25 per cent greater than the full-load current of 
the motor, this will allow momentary overloads of 50 per 
cent without blowing the fuses, because of their time element.* 
This will meet the usual industrial conditions. In some cases 
very severe momentary overloads might require the use of 
larger wires. The rule just given applies to motors for contin¬ 
uous duty.f Table 39 gives the proper size of fuses for motors. 
Crane and hoist motors should be fused for not less than 1.50 
times full-load current. Motors for operating valves, moving 
planer cross-rails, etc., should be fused for 2.0 times full-load 
current. 

Example. A 10-hp., 230-volt motor operating a hoist would re¬ 
quire a current of 1.50 X38 =57 amperes. This would require a No. 5 
rubber-covered wire. 

A fuse must be used in each wire of a branch circuit. When a 
branch circuit is protected by a circuit breaker, it must not be 
set more than 30 per cent above the rating of the wire (Table 
36), unless fuses are also provided. 

316. Determining Size of Feeders or Mains and Fusing. In 
the case of lighting circuits, the feeders or mains should be of 
sufficient size to carry the maximum load as calculated in par¬ 
agraph 312. Table 36 is used in selecting the size. Feeders 
or mains should be fused to the full current capacity allowed by 
this table even if the normal load is considerably less than this, 
as occurs when the size of the circuit is increased to reduce the 
voltage drop. By fusing in this manner there is less chance 
of the fuse blowing because of overloads. When circuit 
breakers are provided, they should be large enough to carry 
the maximum load on the circuit. Since they can always be set 
for currents about 60 per cent above their rating, this allows 
sufficient margin, in general, for possible momentary overload. 
The neutral wire of a three-wire circuit must be of sufficient 

* See paragraph 282. t See paragraph 165. 


316 


CALCULATION OF D.C. SYSTEMS [CHAP. 19 


capacity to carry the entire load of one side of the system 
(Rule 16 h). Generally the neutral wire is made the same size 
as the outside wires, but if these are increased in size to reduce 
the drop, the Code does not require an increase in the size of 
the neutral. If the neutral is grounded,* the fuse in the neutral 
wire must be omitted (Rule 236), except in two-wire branch 
circuits. This is done to prevent an excessive voltage on the 
lamps caused by a neutral fuse blowingf and also to ensure that 
the entire system shall always be grounded while in use. For 
motors, the load based on running conditions, as determined in 
paragraph 314, can be used as a basis for the size of feeder. The 
exceptions to this are the cases where there is one very large 
motor, or where the feeder supplies cranes or similar apparatus. 
In such cases, the starting loads must be considered. 

Example 1. For the example in paragraph 312, Feeder A has an 
estimated maximum load of 330 amperes and hence should be at 
least 400,000 c.m., if rubber insulation is used. This wire is rated 
at 325 amperes and would have to be fused for that current. Since 
the actual load is only 216 amperes there is still plenty of margin allowed 
for extensions. Feeder B would be the same size. 

Example 2. For a three-wire system the feeders would be No. 000 
with neutrals of the same size. The fuses would be 175-ampere size. 

For motor circuits the size of feeders and mains is based upon the 
current required for the maximum running load, except when the 
starting loads are heavy. 

Example 3. For Example 1, paragraph 313, the current is 81.7 
amperes. Hence the size of circuit to supply these motors would 
be No. 3 if rubber insulation is used. 

Example 4. For Example 2, paragraph 313, the current is 524.7 
amperes. This would require a 750,000 c.m. cable. 


Calculation of Voltage Loss 

9 

317. Increasing Size of Wire to Reduce Loss. The sizes of 
circuits determined by the aid of Table 36, in a manner previously 
described, are sufficient to carry the load without overheating. 
It frequently happens, however, with long circuits that the 
voltage loss or drop is excessive under these conditions. The 
voltage drop should therefore be calculated and the size of 
wire increased, if necessary. In Table 40, are given values of 
maximum voltage drop, which should not be exceeded. These 
values apply particularly to industrial work. Where there are 
no mains, this part of the drop may be included in the feeder 

* See paragraph 308. t See paragraph 215. 


PAR. 318 ] CALCULATION OF VOLTAGE LOSS 


317 


drop. Incandescent lamps are very sensitive to voltage changes 
and therefore the drop must be kept small. Power circuits 
can have a greater drop, but it should not be too large, partic¬ 
ularly with induction motors, as they do not operate satisfac¬ 
torily at voltages greatly below normal.* 

318. Calculation of Voltage Drop. For a two-wire d.c. 
circuit, the voltage drop or loss can be calculated from the for¬ 
mula: 


Volts drop = 


21.4 X distance in feet X amperes 
cir. mils 


( 1 ) 


The size of wire can be calculated, if the required voltage drop is 
known, by the formula: 


Cir. mils = 


21.4 X distance in feet X amperes 
volts drop 


( 2 ) 


The distance is the length of run from the supply point to the 
load. If the load is concentrated at one point, it is the total 
length of the circuit or one-half the total length of wire used. 

Example 1. Calculate the voltage drop on a two-wire, No. 00 
feeder (133,100 c.m.), 150 ft. long carrying 50 amperes. 


21.4 X150X50 

Voltage drop =- 

133,100 


= 1.2 volts. 


Example 2. Required, the size of w r ire to carry 50 amperes a dis¬ 
tance of 150 ft. with a drop of 1.5 volts. 

21.4X150X50 

Cir. mils =- 

1.5 

= 107,000 c.m. 

Hence No. 0 is the nearest size. 

While any d.c., two-wire circuit can be calculated by means of 
these formulas, it is generally more convenient to use some form 
of chart. The chartf shown in Fig. 232 will be found useful for 
this purpose. If it is desired to find the drop for the feeder 
already calculated, start at 50 amperes on the lower left-hand 
side and follow vertically until this line crosses that for 00 wire; 

* See paragraph 169. 

f This chart is a modification of one devised by R. W. Stovel and N. A. 
Carle (see Electric Journal, June, 1908), and was published by the author in 
Power, May 18, 1915. It is based on the formulas given above. 






318 


CALCULATION OF D.C. SYSTEMS [CHAP. 19 


then pass horizontally to the right to the line marked 150 ft. 
and follow down vertically and read 1.2 volts. To determine 
the size of wire as in the second problem, start with 1.5 volts at 
the lower right-hand side and follow up vertically to the 150-ft. 
line; then horizontally to the left to the line for 50 amperes, 
which also crosses the No. 0 line at this point. For a three-wire 
system the chart requires some modifications. With a balanced 
system, all the drop occurs in the outside wires, and for the 
lamps on one side we have to take into account the drop in only 
one wire. Hence, if we use the current in the outside wire, and a 
distance equal to the length of run as in a two-wire system, then 
the voltage drop determined from the chart should be divided 
by two. 



Fig. 233.—Method of Finding the Load Centre. 

Units all same size and uniformly spaced. 


Example 3. A three-wire system is carrying 50 amperes on each side 
and the length of run is 150 ft. Using the chart, it is found that for a 
two-wire circuit the drop is 1.2 volts. Hence the drop on one side 
of the three-wire system is 0.6 volt. 

Example 4. It is required to find the w T ire size for a three-wire 
circuit to carry the same load as in Example 2. There w r ould be a 
load of 25 amperes on each side of the system, and w;ith a voltage 
loss in each of the outside wires of 1.5 volts, the loss at the lamps 
would be the same as in Example 2. Using the chart, we find the 
size of a two-wire circuit to carry 25 amperes with 3.0 volts drop. This 
requires a No. 6 wire. Hence this No. 6, three-wire circuit transmits 
the same load as the No. 0, two-wire circuit. The drop in voltage 
across the lamps is the same, being 0.75 volt for each of the wdres of 
the two-wire system and 1.5 and 0 volts drop for the outside wire and 
neutral, to which the lamps are connected, in the three-wire system. 

Where the load is distributed along the circuit, the distance used 
in calculating the drop is not the distance to the end of the run 














Size of Wire in B.&S-G age and Circular Mils 


2l.4xDxI 
e - CM - 


Distance in feet (One Way) 



CD> CVJ ^\OCOQ 


cd o 
it) vo oooo 


o 

CD 

CD 

O CD 

CD 

CD 

CD 

LO 

CD 

LO CD 

CD 

CD 

CVI 

CVJ 

to 

tO NT 

LO 

LSD 



Current in Amperes Drop in Volts 

Fig. 232. —Stovel-Carle Wiring Chart for Direct Current Circuits. (Courtesy of Power.) 


(To face page 818). 









































































































































































































































































































































































PAR. 318 ] CALCULATION OF VOLTAGE LOSS 


319 


but to the load centre. The method of finding the load centre 
can be explained by examples: 

Example 5. There are ten 100-watt, 120-volt lamps spaced as shown 
in Fig. 233. The total load is 1000 -M20 =8.33 amperes. The distance 
between end lamps is 67.5 ft. Since the lamps are all the same 
size and evenly spaced, the load can be considered as concentrated at a 
point halfway between the fifth and sixth lamps. The total distance 
• , , , 67.5 

is therefore 35 i-=68 ft. 9 in. This is the position of the load 

2 

centre. With a No. 12 wire the drop is 1.88 volts. 

Example 6. With the lamp loads unevenly spaced and of different 
sizes (Fig. 234), the load centre must be found by multiplying each 
load by its distance from the panel board. The sum of these products 



Units different sizes and unevenly spaced. 


divided by the total load will give the distance of the load centre. 
Thus for Fig. 234 we would have: 

50X100= 5,000 
, 60X200 = 12,000 

65X100= 6,500 
69X100= 6,900 
77X100= 7,700 


38,100 

Totai load =600 watts. 

Distance of load centre =38,100 4-600 =63.5 ft. 

At 120 volts the current is 5 amperes. With No. 14 wire, the drop 
would be 1.65 volts. The drop thus calculated is that to the furthest 
lamp (A in Fig. 234). The drop to the other lamps which are 
nearer the panel board would be somewhat less. It will be noted 
that the load on the tap was calculated as if it were located on the 
main run. If the tap circuit is very long, the drop on this would 























320 


CALCULATION OF D.C. SYSTEMS [CHAP. 19 


have to be added to the drop on the main run to the point where 
the tap is connected, but usually the drop in the tap can be neglected. 

If the loads are given in amperes, the same method of finding the 
load centre can be employed. 

319. Calculating Drop on Branch Circuits. For lighting cir¬ 
cuits, it is necessary to calculate the drop on a few of the longer 
runs only. If the distance to the load centre is less than 
the distances given in Table 41, there is no need of checking the 
drop. For motor circuits, the branch usually carries a single 
motor. The drop should be calculated for the full-load currents, 
using the formula or chart. In the case of motor branch cir¬ 
cuits, it is not generally necessary to calculate the drop for all 
the branches. Usually the size of wire which must be used to 
carry the required current is large enough to keep the drop bfelow 
the values specified in Table 40. 

320. Calculating Drop on Feeders and Mains. After the 
loads and smallest size of wire have been calculated,* the drop 
should be determined. For a feeder which supplies several 
panel boards, the load centre could be found and the drop to 
the farthest panel board determined thereby. This would serve 
as a check, but it is usually better to find the drop to each panel 
board on the feeder. To do this, the drop for each part of the 
circuit is found by means of the formula or chart. The total 
drop to any point can then be found by adding the drop on 
each part. For lighting circuits, the load used in calculating 
the drop is the actual load and not the maximum load.f 

Example. Taking the loads used in Example 1, paragraph 312, 
and the sizes of feeders calculated in paragraph 316, determine the drop, 
if a two-wire, 120-volt system is used. Feeder B (400,000 c.m.) carries 
210 amperes, 170 feet. This would give a drop of 1.91 volts. Next, 
the drop on the mains should be calculated. The main feeding 
panel No. 6 must be at least No. 00 (for 140 amperes). The length 
is only 12 ft. and the drop is 0.17 volt. The total drop to panel 
No. 6 is therefore 2.08 volts. The allowable drop is 5.2 volts, hence 
the sizes are large enough. Feeder A is only 130 ft. long and carries 
216 amperes. The drop is 1.5 volts, which is satisfactory. 

For motors, the actual load with the motors running should be 
used to calculate the drop. Thus, in Example (1) paragraph 313, 
the drop should be figured for a current of 81.7 amperes. The 
drop when a motor is starting may be greater than this, but as 
it only lasts for a short time the larger drop is not serious. 

* See paragraph 316. t See paragraph 312. 


CHAPTER 20 


CALCULATION OF A.C. SYSTEMS 

321. In a.c. circuits, due to the reversals of the current, 
phenomena occur which are not present in d.c. systems. Of 
these, the only ones which are important in interior wiring cal¬ 
culations are self-induction and skin effect. 

322. Self-Induction. A wire carrying alternating current is 
surrounded by a magnetic field which is also alternating. This 
field generates in the wire a voltage which opposes the flow of 
current through the wire. In an a.c. circuit, therefore, there is a 
drop due to the resistance of the wire (the same as d.c. drop), 
and an additional drop due to the self-induction. The amount 
of inductive drop depends upon the spacing of the wires, the 
frequency and the size of the wire. If the wires forming a cir¬ 
cuit are close together, as in a multiple cable or when installed 
in the same conduit, the field around each wire is almost neu¬ 
tralized by the current in the other wires and the inductive drop 
is small. On the other hand, exposed wiring run on cleats and 
having a separation of several inches may sometimes have an 
inductive drop double the d.c. drop. The frequency affects the 
inductive drop, because faster reversals of the field produces a 
higher self-induced voltage. For 60 cycles, the inductive drop 
is 2.4 times the drop for 25 cycles. Where the power factor of 
the load is 1.0 (as for incandescent lamps or heaters), the induc¬ 
tive effect is usually not important. For arc lamps or motors 
however, it generally cannot be neglected. 

323. Skin Effect. This term is given to the apparent increase 
in the resistance of a conductor when carrying alternating cur¬ 
rent. The central portion of a wire is surrounded by a magnetic 
field which is stronger than the field around the outside portion. 
Hence the inductive voltage produced in the central portion is 
higher, giving a greater opposition to the flow of the current 
through the centre. As a result, the current is crowded towards 
the surface of the conductor, to such an extent that the centre, 

321 


322 


CALCULATION OF A.C. SYSTEMS [CHAP. 20 


in the case of large conductors, carries practically no current. 
The wire acts as if the cross-sectional area had been reduced by 
taking out the central part; in other words, the apparent re¬ 
sistance is increased. Skin effect increases with the frequency 
and the size of wire. Table 42 can be used to determine skin 
effect. It will be seen that this is negligible for wires smaller 
than 300,000 cir. mils on 60 cycles and 750,000 cir. mils on 
25 cycles. The skin effect for iron conductors, such as steel 
rails, etc., is very much greater. It is apparent that, due to 
skin effect, a large cable carrying alternating current would run 
somewhat warmer than when carrying the same amount of 
direct current. Although the Code makes no difference .in the 
current rating,* this point should be remembered when very large 
cables are to be used for carrying alternating current. 

324. Power Factor. In a.c. circuits, we have to consider two 
quantities, the real power (expressed in watts or kilowatts), 
and the apparent power (expressed in volt-amperes or kilo¬ 
volt-amperes). The apparent power is the product of the volts 
and amperes. The real power may equal the apparent power, 
but in many cases it is less. It can never be greater. The real 
power is measured by an instrument called a wattmeter. The 
ratio: Real power divided by the apparent power gives the 
power factor. Hence, we can say that the power factor is the 
quantity by which the apparent power is multiplied to obtain 
the real power. The usual values of the power factor are given 
in Table 43. All of these, except the first are “ lagging ” 
power factors. “ Leading ” power factors may be produced 
by synchronous motors, but they will not be considered here. 
The effect of a lagging power factor is, in general, to increase 
the total voltage drop in an a.c. circuit. 

325. Grouping of Conductors. With direct current, it makes 
no difference (as far as drop is concerned) whether all the wires 
of a circuit are in the same conduit or are separated. A.c. cir¬ 
cuits, however, when installed in iron conduit, must have all the 
wires of a circuit in the same conduit. If separated, the mag¬ 
netic fields around the wires are not neutralizedf and, in fact, 
they are greatly increased by the presence of the iron. As 
a result, the inductive drop will be very high and the con¬ 
duit will heat if the wire is carrying a large current. If the 
current is so great as to require more than one wire for each lead 

* See Table 36. t See paragraph 322. 


par. 325 ] 


GROUPING OF CONDUCTORS 


323 


of the circuit, and it is not feasible to put them all in one con¬ 
duit, because of the large size required, the leads should be 
divided into two or more groups, each containing all the poles 
of the circuit. The proper arrangement for a three-phase cir¬ 
cuit is shown in log. 235, where the leads of the three phases are 
1, 2 and 3, respect¬ 
ively, 1 a and lb being 
of the same polarity. 

This rule applies for 
all types of a.c. sys¬ 
tems except the two- 
phase, four-wire, 
which is practically 
the same as two 
single-phase circuits, 
so that phases A and 
B may be run in 

separate conduits. The relative amount of inductive drop 
increases rapidly as the size of conductor is increased. As a 
rule, wires larger than 300,000 cir. mils should be avoided, 
except where the wires are in conduit, when a size of 500,000 
cir. mils or larger may be used. Circuits requiring a greater 
capacity than this can best be made up of two or more wires 
in parallel. 




Fig. 235.—Arrangement of Wires in 
Conduit. 


Example. A 500,000 cir. mil feeder (spacing 6 ins.) carrying a 
load of 300 amperes at 60 cycles a distance of 500 ft. has a drop 
of 18.5 volts with a power factor of 0.8. It is required to find the 
size of wire, to give the same drop, with the load divided between 
two feeders. By calculation, it is found that a No. 00 (133,000 cir. 
mils) circuit carrying 150 amperes gives a drop of 17.2 volts. Hence 
two of these could carry as much load as the single 500,000 cir. mil 
feeder, and the drop would be somewhat less. Therefore the load 
can be transmitted by two circuits totaling 266,000 cir. mils with 
practically the same voltage drop as a single 500,000 cir. mil cir¬ 
cuit. The saving in copper is partly offset by the additional cost 
of running two circuits. 


If two feeders are used, they should always be of the same size. 
Thus, in the example given, if a No. 000 (168,000 cir. mils), 
cable and a No. 0 (106,000 cir. mils) cable were used, the smaller 
wire would be overloaded, although the total amount of copper 
is 274,000 cir. mils. 


324 


CALCULATION OF A.C. SYSTEMS [CHAP. 20 


Calculation of Load on a Circuit 

326. As in d.c. circuits, both the maximum and actual loads 
on lighting panel boards, feeders and mains should be deter¬ 
mined. For motors, running and starting conditions must be 
figured, using demand factors as explained in paragraph 313. 

327. Single-phase Circuits. For incandescent lamps or 
heaters on two-wire systems, the load is found in the same way 
as for direct current.* For arc lamps, the total current can be 
found by adding together the currents required for the various 
lamps. If the rating is given in watts the current is found by 
dividing the watts by the voltage and the power factor (see 
Table 43). 

Example 1. Find the current required for ten arc lamps each 
taking 500 watts at 110 volts. 

10X500 

Current =-— =75.7 amperes. 

110X0.60 

Where loads of different power factors (for example arc and in¬ 
candescent lamps) are on the same circuit the total current is 
slightly less than the sum of the currents required by the two 
kinds of load. In general, however, this difference is so slight 
that it can be neglected, f 

Example 2. For 10 flame-arc lamps taking 7.5 amperes each and 
15 Cooper Hewitt lamps requiring 3.8 amperes, we have 

Flame arcs 10 X7.5 =75 amperes 

Cooper Hewitt lamps 15X3.8 =57 

Total 132 amperes 

Actually, since the power factors are different, the total current 
is less than 132 amperes. It is, in fact, 130 amperes. J 

The full-load current of single-phase motors can be found by 
doubling the values given in Table 23. The starting current 
may be taken as 2 times the full-load current. The total run¬ 
ning current and starting current can b.e calculated by the 
methods given in paragraph 313. 

* See paragraph 310. 

f See paragraph 330 for method of calculation. 

X See paragraph 330. 




par. 328 ] 


CALCULATION OF LOAD 


325 


Example 3. 110-volt, single-phase motors. 

Running conditions: 


1-5 hp. motor 
1-3 

1- 2.5 

2 - 1 

1-3 (spare) 


46.4 amperes 
28.8 

25.0 

22.4 
28.8 


Total 151.4 amperes 

Running current 151.4 X0.65 =98.5 amperes. 

Single-phase, three-wire circuits are arranged the same as 
d.c., three-wire circuits and the calculations are the same 
except for the modifications due to having different power fac¬ 
tors. 

328. Three-phase Circuits. Two arrangements of lamps or 
other single-phase loads are possible (Figs. 113 and 114). The 
branch circuits supplying the lamps would be ttvo-wire and 
would be connected to the three-phase feeders through three- 
phase panel boards in such a manner as to distribute the load 
as evenly as possible on the three phases. The total current 
taken by the branch circuits connected across one phase can be 
calculated as if they were on a single-phase system.* When 
a three-phase, three-wire system is used it is apparent that 
there are two of these single-phase groups connected to each 
of the line wires. If the current required by all three groups is 
the same (balanced load), the current in each of the three-phase 
line wires is 1.73 times the current taken by one of the groups. 


Example 1. Sixty 100-watt, 120-volt lamps are connected to a three- 
phase, three-wire feeder. The current taken by each phase is 


20X100 

-= 16.7 amperes. 

120 


The current in each line wire is 16.7 Xl-73 =28.9 amperes. 

Example 2. With the same load as in Example 1, paragraph 311, 
we have for each phase: 

Lamp load 36 X100 =36d0 watts 
Portables 9 X40 = 360 


Total 3960 watts 


* By the methods given in paragraph 327. 





326 


CALCULATION OF A.C. SYSTEMS CHAP. 20 


Single-phase current 3960 4-120 =33 amperes. 

Actual current in three-phase line 1.73 X33 =57 amperes. 
For maximum load conditions we have: 

Lamp circuits 4 X1200 =4800 watts 
Portable circuit 1 X600 = 600 
Spare circuit 1 X600 ==\ 600 


Total 6000 watts 

Single-phase current 6000 4-120 =50 amperes. 

Maximum current in three-phase line 1.73 X50 =86.5 amperes. 

If the loads on the three phases are not equal, the system is 
unbalanced and the currents in the three line wires are not 
alike. The current in any line may be found as follows: Divide 
one of the loads by 2 and add the result to the other load con¬ 
nected to the same line wire. Call this result A. Multiply the 
first load by 0.866 and call this B. Square A and B, add them 
and extract the square root. This is the current in the line. 

Example 3. The feeder in Fig. 1136 has loads of 20, 10 and 20 

amperes connected across the three phases. The total load for 

line 1 is a combination of 20 amperes and 10 amperes. Hence, 

A =20+10 4-2=25 
5=10X0.866 =8.66 

The current in 1 is therefore: — "C (25) 2 +(8.66) 2 =26.5 amperes. 

The load for line 2 is a combination of 20 amperes and 20 amperes. 

Hence, 

A =20+20 4-2 =30 
B =20 XO- 866 =17.32 

The current in 2 is therefore: — (30) 2 +(17.32) 2 =34.6 amperes. 

The load for line 3 is a combination of 10 amperes and 20 amperes. 

Hence, 

A =10+20 4-2 =20 
B =20 X0. 866 =17.32 

The current in 3 is therefore: — \/( 20)2 + (17.32) 2 =26.5 amperes. 

This method is correct only when the loads on the different 
phases all have the same power factor. When this power factor 
is different, the method gives only approximate results for the 
reasons stated in paragraph 330. When a three-phase, four- 
wire system is used, the lamps are connected between the line 
wires and neutral (Fig. 114). The current in each line wire is 
the total current required for the load connected to that wire. 
If the loads are equal (balanced), the currents in the line wires 






par. 329 ] 


327 


CALCULATION OF LOAD 

i 

will be alike, and no current will flow in the neutral. With 
unequal loads, there will be a neutral current. When there is 
a load on one phase only the neutral must carry the same 
current as the line wire. Hence, all three line wires and the 
neutral are usually made the same size. Motors would be 
connected to the three line wires for either of the three-phase 
systems and would have no connection with the neutral (Figs. 
113 and 114). Table 22 can be used to determine the full-load 
current of each motor. The running and starting conditions 
should be calculated for motor circuits as explained in para¬ 
graph 313. The starting current of squirrel-cage motors can 
be taken as 2 times the full-load current and for slip-ring motors 
1.25 times the full-load current. The actual current is more 
than this, but it lasts for such a short time that the overload 
capacity of the fuses will prevent their blowing. 

Example 4. For three-phase, squirrel-cage motors (220-volt) we 
have, using Table 22: 

1-5 hp. motor 13.4 amperes 

1-3 8.2 

1- 2.5 7.3 

2- 1 6.4 

1-3 (spare) 8.2 


Total 43.5 amperes 

Running load 43.5 X0.65 =28.3 amperes. 

Example 5. If in addition there was a 50-hp. motor, we should 
have to consider the starting conditions as follows: 

Running load, six motors = 28.3 amperes 

Starting load 50-hp. motor 122X2=244.0 

Total =272.3 amperes 

329. Two-phase Circuits. Lamps or other single-phase loads 
would be distributed on the two phases. A four-wire, two-phase 
system (Fig. 115) can be treated like two single-phase systems 
and calculated in the same manner. A three-wire, two-phase 
system (Fig. 116) would have the lamps connected between the 
outside wires and the common wire. The current in an outside 
wire is the total current required by the load connected to that 
wire. The current in the common wire, for a balanced load, is 
found by multiplying the current in either line wire by 1.41. 




328 


CALCULATION OF A.C. SYSTEMS [CHAP. 20 


For an unbalanced load, the current in the common wire is 
found by squaring the value of current in each outside wire, 
adding these results and extracting the square root. Where the 
loads on the two sides have different power factors, these methods 
give only approximate results. 

Example 1. Four-wire system. Sixty 100-watt, 120-volt lamps. 

With equal loads on the two phases there would be 

30X100 

-=25 amperes 

120 

in each of the four wires. 

Example 2. Three-wire system (Fig. 116a), balanced load. 

Current in each outside wire =30 amperes. 

Current in common wire 30 XI.41 =42.3 amperes. 

Example 3. Three-wire system (Fig. 1166), unbalanced load. 

Current in line 1 =15 amperes 

Current in line 2 =30 

Current in common wire—V / ( 15) 2 +(30) 2 =33.5 amperes. 

Motors are connected to all of the wires of the circuit. Table 23 
gives the currents required for two-phase motors. Both the 
running and starting conditions should be calculated as ex¬ 
plained in paragraph 313. The starting current can be taken as 
the same percentage of full-load as for three-phase motors; i.e., 
2 times full-load current for squirrel-cage motors and 1.25 
times full-load current for slip-ring motors. 

Example 4. Four-wire, 220-volt system, squirrel-cage motors: 


1-5 hp. motor 

11.6 amperes 

1-3 

7.2 

1-2.5 

6.3 

2-1 

5.6 

1-3 (spare) 

7.2 

Total 

37.9 amperes 


Running load 37.9 X0.65 =24.6 amperes. 

Example 5. If in addition there was a 50-hp. motor, we should 
have to consider the starting conditions as follows: 

Running load, six motors = 24.6 amperes 

Starting load 50 hp.-motor 105X2=210.0 


Total 


= 234.6 amperes 





PAR. 330 ] DETERMINING SIZE OF A CIRCUIT 


329 


Example 6. For a three-wire, 220-volt system, Example 4 above: 

Current in each outside wire =?24.6 amperes 

Current in common wire 24.6X1.41 =34.7 

Example 7. For load conditions as in Example 5 above: 

Current in each outside wire =234.6 amperes 

Current in common wire 234.6X1.41 = 330 


330. Combining Loads having Different Power Factors. The 

methods used in the previous paragraphs for finding the com¬ 
bined loads are strictly accurate only when the entire load has 
the same power factor. The power factors of various loads are 
given in Table 43. The error in using the approximate method 
is greatest when there is a large difference in power factor. 
Since, however, it gives values which are too high, the error is 
on the safe side. In applying the more accurate method, the 
current required for loads having the same power factor is first 
found. Each current thus determined is multiplied by a 
“ reactive factor ” and “ resistance factor ” taken from Table 44. 
The total resistance and reactive parts are then found by addi¬ 
tion. The actual current is found by squaring each of these 
sums, adding the results and extracting the square root. 


Example. Referring to Example 2 in paragraph 327: 


Resistance part 

Flame arcs 75X0.60 =45 

Cooper Hewitt lamps 57X0.85 =48.5 


Reactive part 
75 X0.80 =60 
57 X0.53 =30.2 


93.5 

Total current V(93.5) 2 +(90.2) 2 = 129.9 amperes. 


90.2 


The sum of the two currents is 132 amperes, so it will be seen 
that the error in using the approximate method is not great. 

331. Determining Size of Branch Circuits and Fusing. The 
size of branch lighting circuits is determined in the same way 
as for d.c. circuits.* For motors, the branch circuits must be 
large enough to carry a load at least 25 per cent greater than the 
full-load current (Rule 86). The wires for squirrel-cage induc¬ 
tion motors must be made larger than this because of the heavy 
starting current. Tables 22 and 23 give the full-load current 
required. A squirrel-cage motor when starting under full-load 
torque requires about 70 per cent of normal voltage and takes 

* See paragraph 315. 






330 


CALCULATION OF A.C. SYSTEMS [CHAP. 20 


from 4 to 5 times full-load current from the line. This current 
lasts for only a few seconds, however, so that, owing to the over¬ 
load capacity of enclosed fuses, the branch circuit can be fused 
for from 2.5 to 3 times full-load current. When no starter is 
used the fuses should be from 3 to 3.5 times the full-load cur¬ 
rent. Owing to the heavy starting current required for squirrel- 
cage motors, the Code allows the branch circuits to be pro¬ 
tected in accordance with column B of Table 36 even when 
rubber insulation is used (Rule 23e). If this was not done, the 
cost of the branch wiring would be greatly increased. 

Example 1. A 20-hp., 220-volt, three-phase motor has a full-load 
current of 54.6 amperes. The wire must be large enough to carry at 
least 25 per cent overload, i.e., 68.2 amperes. The starting fuses 
must be rated at about 2.7X54.6 =148 amperes. If 150-ampere 
fuses were used, with rubber-insulated wire, the regular rules for 
fusing would require a No. 00 wire. Because of Rule 23e, however, 
we can use the rating for weatherproof wire, and hence a No. 1 wire can 
be used. Since the starting current does not last long enough to 
overheat the wire, this size can safely be used even where insulated 
with rubber, as its normal carrying capacity is 100 amperes. This 
would allow the motor to carry about double full-load current when 
running. 

The wire sizes given in Tables 22 and 23 are determined in 
this manner. For wires with insulation other than rubber the 
same allowance is not made; that is, according to the Code, the 
wire must be chosen in accordance with column B also. How¬ 
ever, the inspectors will often allow induction-motor wires, 
when exposed, to be fused somewhat higher than the values 
given in column B. It is apparent that the wires will be ade¬ 
quately protected from injury when fused in accordance with 
these rules, but the motor will not be properly protected against 
continuous overloads, which would not cause the fuses to blow, 
but still would be larger than the motor could safely stand for 
any length of time. For this reason squirrel-cage motors should 
always be provided with running fuses which are cut out during 
the starting period by means of the starting switch or compen¬ 
sator.* The ordinary induction motor is rated to stand a 25 
per cent overload for tw r o hours, but it can carry greater over¬ 
loads for shorter periods. Therefore the running fuses should 
hav^e a rating about 50 per cent greater than the full-load current 
of the motor. The slip-ring induction motor takes about 

* See paragraph 183. 


PAR. 332 ] CALCULATION OF VOLTAGE LOSS 


331 


full-load current for full-load torque.* The size of wire is 
therefore determined by the allowable overload, which should 
be 50 per cent as before. With these motors, however, the size 
of wire, when rubber insulation is used, would be determined 
from Column A of Table 36. 

Example 2. A 20 hp., 220-volt, three-phase, slip-ring motor would 
require 82 amperes -when running at 50 per cent overload. For 
rubber insulation the wire should be No. 3, or for slow-burning insula¬ 
tion No. 5. The running fuses can be determined by the aid of 
Table 39. 

332. Determining Size of Feeders or Mains and Fusing. 
For lighting circuits the maximum load is used to determine the 
wire size as in d.c. systems. For motor circuits, the wire Size is 
usually determined for the maximum running load.f Rubber- 
covered wire used for motor feeders or mains is selected from 
Column A of Table 36 and not from column B, as is allowed for 
branch circuits. 


Example 1. Single-phase circuits, rubber insulation: 
Example 3, paragraph 327, No. 1 wire is required. 


Example 2. Three-phase circuits: 
Example 2, paragraph 328, 
Example 4, paragraph 328, 
Example 5, paragraph 328, 

Example 3. Two-phase circuits: 
Example 4, paragraph 329, 
Example 5, paragraph 329, 
Example 6, paragraph 329, 


No. 2 wire is required. 

No. 8 wire is required. 

300,000 cir. mil cable is re¬ 
quired. 

No. 10 wire is required. 

No. 0000 cable is required. 

No. 10 wire is required for each 
outside lead, No. 8 wire for 
common. 


Calculation of Voltage Loss 

333. Increasing Size of Wire to Reduce Loss. As in d.c. 

systems, the circuits must frequently be made larger than the 
minimum size calculated in the previous paragraph, to keep the 
voltage loss at a reasonable figure. The values of allowable loss 
are given in Table 40. In calculating the drop on a.c. circuits, 
the d.c. chart can be used (with slight modifications, to suit the 
different systems), provided the sizes of circuit do not exceed 
the following: 

* See paragraph 167. t See paragraphs 314 and 316. 


332 


CALCULATION OF A.C. SYSTEMS [CHAP. 20 


Kind of Load. 

Size of 

Circuit. 

In Conduit. 

2.5-in. Spacing. 

Incandescent lamps—60 cycles. . . 

No. 0000 

No. 00 

Incandescent lamps—25 cycles. . . 

600,000 c.m. 

400,000 c.m. 

Induction motors—60 cycles. 

No. 1 

No. 3 

Induction motors—25 cycles. 

No. 0000 

No. 00 

Arc lamps—60 cycles. 

No. 0 * 

No. 1 

Arc lamps—25 cycles. 

300,000 c.m. 

No. 0000 


The method of calculation for larger circuits will be taken up in 
the succeeding paragraphs. As a rule, in calculating the drop on 
a lighting circuit the actual load is used, and not the “ maximum 
load ” based on allowing 600 watts per branch. The maximum 
load is used only in determining the smallest size of wire which 
can be employed, as described in paragraph 332. For motors 
the methods of calculating the drop, as just described, can also 
be employed. The current used is the total current under run¬ 
ning conditions, and not as a rule the starting current. Thus, 
in example (4) paragraph 328, the size of wire is determined by 
the running load, 28.3 amperes and, therefore, a No. 8 wire is 
used if the insulation is rubber. The drop would be calculated 
for 28.3 amperes at a power factor of 0.80. In the case of exam¬ 
ple (5), paragraph 328, the size of wire chosen (300,000 cir. mils), 
is fixed by the starting load, 272.3 amperes. The drop should 
be calculated for the running conditions, which, in this case, 
would be: 


1—5 hp. motor 

13.4 

1-3 

8.2 

1-2.5 

7.3 

2-1 

6.4 

1-50 

122.0 

1—3 (spare) 

8.2 


Total, 165.5 amperes 

Running load 165.5X0.65 = 108 amperes. If it is suspected 
that the drop, when motors are starting, is excessive, this can 
also be calculated. It is satisfactory, however, to allovV a 















PAR. 334] CALCULATION OF VOLTAGE LOSS 


333 


somewhat greater drop than for running conditions. This 
should not exceed a total of more than 15 per cent for the entire 
loss between service point and motor terminals. 

334. Single-phase Circuits. When the inductive effect is 
neglected, the calculations are exactly the same as for a d.c., two- 
wire system, and the voltage loss can be determined either by 
the formula * or the chart. If the circuit is larger than the 
sizes given in the previous paragraph, so that the inductive effect 
cannot be neglected, the drop should first be found as for direct 
current. This value of drop is then multiplied by a “ drop 
factor ” which is found from Tables 45 and 46. The additional 
drop due to inductance depends upon the ratio of the reac¬ 
tance to the resistance and also upon the power factor. The 
reactance depends upon the frequency and the distance between 
the wires. Table 45 gives these ratios for the three frequencies 
in common use and for wires run in conduit or spaced the usual 
distances apart. The reactance ratio found from Table 45 for 
the particular frequency and spacing is then used in connection 
with Table 46 to find the drop factor. Multiplying the d.c. 
drop by this drop factor gives the a.c. drop. 

Example 1. A two-wire feeder 150 ft. long is carrying a load of 
incandescent lamps totaling 50 amperes at 120 volts. The wires 
are in conduit and the frequency is 60 cycles. The smallest wire 
which could be used would be No. 6 (Table 36). The drop should 
not exceed 2.5 per cent (Table 40) or 3.0 volts. From the chart, 

Fig. 232, we find this would require a No. 3 wire. By Table 45, the 
reactance ratio is 0.22, and for a power factor of 1.0 (Table 46), 
the drop factor is 1.00. Hence the a.c. drop is 3.0 volts and No. 3 
wire should be used. 

For loads at different power factors an average can be used if the 
loads are of about the same size. If one of the loads is much 
larger than the others, the power factor of this load should be 
used. 

Example 2. For the flame-arcs (power factor 0.60) and the Cooper 
Hewitt lamps (power factor 0.85) in Example 2, paragraph 327, a power 
factor of 0.70 may be used. For 130 amperes, nothing smaller than a 
No. 00 wire can be used. The frequency is 60 cycles, the spacing 
2.5 ins. and the length of the circuit 100 ft. The d.c. drop is 2.1 volts. 

The ratio is 0.80 and the drop factor 1.27. Hence the a.c. drop is 
1.27X2.1 =2.67 volts. As this is less then 2.5 per cent (3 volts) the 
wire size is satisfactory. 


* See paragraph 318. 


334 


CALCULATION OF A.C. SYSTEMS [CHAP. 20 


This method of calculating drop on a line carrying several loads 
at different power factors is not strictly accurate, but in most 
cases is close enough. A more accurate method would be to 
calculate the drop due to each part of the load separately 
and then to combine them with the aid of the factors given in 
Table 44. 

Example 3. Applying this to Example 2, using a No. 000 feeder, 
we have: t 

Flame arcs, d.c. drop, 0.96 volt, a.c. drop for 0.60 power factor, 1.32 


volts. 

Cooper Hewitt lamps, d.c., drop, 0.73 volt; a.c. 
factor, 1.0 volt. 

Combining these: 

drop for 0.85 power 

Resistance Part. 

Reactive Part. 

Flame arcs, 1.32X0.60=0.792 

Cooper Hewitt lamps, 1.00 X0.85 =0.85 

1.32X0.80=1.056 
1.00X0.53= .53 

1.642 

1.586 

Total drop=”v 1.642 2 +1.586 2 =2.28 volts. 



This method always gives slightly lower values than the approx¬ 
imate method. The calculations for motor circuits are made 
in a similar manner. 

335. Three-phase Circuits. For a three-wire, three-phase 
system, the drop in one wire is first calculated. If the currents 
are the same in all the wires, the total drop is 1.73 times the drop 
in one wire.' 

Example 1. Referring to Example 1, paragraph 328, the smallest 
size of wire which could be used is No. 8, rubber covered. With 
exposed wiring, 60 cycles and a length of run of 100 ft., we find the 
drop from the chart to be 3.75 volts for two wires. The drop on one 
wire is therefore 1.87 volts. The a.c. drop on one wire is also 1.87 
volts because the inductive effect can be neglected.* The total drop at 
the lamps is 1.73X1.87=3.24 volts. To keep the drop within 2.5 
per cent the size would have to be increased. 

Example 2. For the load given in Example 2, paragraph 328, a 
No. 2 wire is the smallest size allowable. The drop on a three-phase 
feeder of this size and 14 ft. long f carrying 57 amperes is 1.73 X0.13 
= 0.23 volt, since the inductive effect can be neglected. 


* See paragraph 333. 


t See Fig. 231. 












PAR. 335 ] CALCULATION OF VOLTAGE LOSS 


335 


For a four-wire, three-phase system, the total loss across each 
lamp load is equal to the loss in the wire supplying that load, 
since there is n loss in the neutral as long as the load is bah 
anced. 

Example 3. With sixty 100-watt lamps divided among the three 
phases, the current is 16.7 amperes in each line wire. If a No. 12 
wire is used to carry this current & distance of 100 ft. the d.c. drop 
is 5.47 volts for two wires. The actual drop at the lamps is there¬ 
fore 2.73 volts. 

When the load is unbalanced, there would be some additional 
loss due to the drop in the neutral, but in general this is so small 
that it can be neglected. Since motors are connected only to 
the line wires, the method of calculation is the same for either 
system. The drop in one wire is figured as just described and 
this is multiplied by 1.73 to give the total drop across each 
phase. 

Example 4. Referring to Example 4, paragraph 328, the smallest 
wire size was found to be No. 8 (paragraph 332). The feeder is 300 
ft. long and in conduit. Frequency, 60 cycles. By referring to the 
tabulation in paragraph 333, it will be seen that the inductive effect 
can be neglected. Using the chart, the drop for a two-wire circuit 
300 ft. long is found to be 11 volts. Hence for one wire it is 5.5 volts 
and between phases it is 5.65X1.73=9.5 volts. The allowable drop 
is 6 per cent or 13.2 volts (Table 40). Hence No. 8 wire is satis¬ 
factory. 

Example 5. Referring to Example 5, paragraph 328, the wire size 
is 300,000 cir.mils (paragraph 332). The feeder is 300 ft. long and in 
conduit. The power factor, determined from the 50 hp. motor, would 
be 0.85 (Table 43). The maximum running current is 108 amperes 
(paragraph 333). By the chart, the drop on a two-wire circuit is 
found to be 2.3 volts. Hence the three-phase drop is 1.15 XI.73 =1.98 
volts. The frequency is 60 cycles and hence the inductive effect cannot 
be neglected (paragraph 333). The ratio of reactance to resistance 
is 1.01 (Table 45). The drop factor for a power factor of 0.85 is 1.39 
(Table 46). Hence the drop is 1.98X1.39 =2.75 volts, which is less 
than the maximum allowable drop. The size need not, therefore, 
be increased. 

When the wires are run exposed, they are usually installed 
side by side and hence the spacing between wires varies. Thus 
if a spacing of 2.5 in. between adjacent wires were used, the dis¬ 
tance between the outside wires would be 5.0 in. As a result, 
the drop on the two outside wires would be somewhat greater 
than the drop on the middle wire. The average drop on the 
wires can be approximately determined by calculating for a 


336 


CALCULATION OF A.C. SYSTEMS [CHAP. 20 


spacing 50 per cent greater than the distance between the centre 
and either outside wire. Thus, for wires 6 in. apart, calculate 
the drop on a wire assuming the return 9 in. away. This drop 
multiplied by 1.73 will give the approximate drop between 
phases. If the wires are transposed, that is, if each wire is in 
the centre for one-third the distance, use a spacing 26 per cent 
greater than the spacing between adjacent wires.* 

336. Two-phase Systems. In a four-wire two-phase system, 
each phase can be calculated independently like single-phase 
circuits, f For a three-wire, two-phase system the drop in the 
common wire causes an unbalancing effect upon the voltages, 
but for interior wiring this is not serious. The calculation of 
drop for this system is too complicated to be given here. An 
approximate result can be obtained by calculating the drop on 
the outside wire in the regular way. To this can be added 
0.707 times the drop in the common wire. 

* H. B. Dwight, Electric Journal, July, 1915. 

f See paragraph 334, 


I 


CHAPTER 21 


EXAMPLES OF WIRING SYSTEMS 

Wiring System for an- Office Building 

337. There are seventeen floors above the street level and 
two below. The first four floors and the basement are used for 
exhibition, sales rooms and executive offices, while the remainder 
of the floors are used for ordinary offices. Fig. 236 shows the 
arrangement of a typical office floor. 

338. Power Supply. Both lighting and power service is sup¬ 
plied by a steam-driven plant in the sub-basement. The ser¬ 
vice is 240-120 volts, three-wire, direct current. Three-wire 
Edison service is also provided through supply feeders 1 and 2. 
All the wiring is in rigid conduit concealed in walls and floors. 
Lamps are supplied at 110 volts and motors at 220 volts. The 
switchboard consists of three generator panels, two motor panels 
and three lighting panels. 

339. Feeder System. There are 22 lighting feeders and 11 
power feeders. The lighting feeders are three-wire with neutral 
the same size as the outers and the power feeders are two-wire 
(Figs. 237-8). These feeders are divided between two rising 
points near either end of the building. Feeders A and B supply 
lighting service to the offices and feeders C supply stair, corridor 
and toilet lighting. The elevators are supplied on feeders sep¬ 
arate from those for the ventilating fans, etc. The small ven¬ 
tilating fan motors are supplied at 110 volts from panels A and 
B on the second and third floors. Each lighting feeder supplies 
three panel boards, one above and one below the floor where the 
feeder terminates. Three-pole switches are provided in the 
middle panel, through which the mains to the other two panels 
are supplied. In some cases, feeders C supply five floors instead 
of three. 

340. Panel Boards. The lighting panel boards (Fig. 237), 
have two-wire ; 110-volt branches with 10-ampere knife switches 

337 


On 6 T " Floor only 


338 


EXAMPLES OF WIRING SYSTEMS 


[chap. 21 



Fig. 236.—Wiring for an Office Building. Floor Plan. 

(Mr. C. E. Knox, Consulting Engineer.) 















































































































































































































































































































Fig. 237. —Wiring for an Office Building. Riser diagram. To face page 339, 

(Mr. C. E. Knox, Consulting Engineer.) 


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par. 340] 


OFFICE BUILDING 


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395 

34 

0000 

0000 

292 

I3\76th 

C 

12 

30 

396 

25 

. 2 

2 

2" 

14 

R 

4 

20 

50 

440 

36 

00 

00 

252 

15 

R 

C 

20 

50 

500 

4 

00 

00 

25: 

16 

B 

B 

34 

85 

120 

3 3 

2 

2 

2" 

n 

A th 

B 

42 

705 

200 

34 

00 

00 

25? 

18 ■ 7 th 

B 

44 

HO 

250 

3. 

00 

00 

29:' 

I9'l0 fjr 

B 

42 

105 

296 

33 

00 

00 

- 

2043 th 

P 

42 

105 

340 

3. 7 

000 

000 

-£K- 

2!'!6 th 

B 

42 

705 

390 

3 5 

000 

000 

29? 

2? 

R~~ 

6 

20 

50 

415 

33 

00 

00 

29.2 


Fig. 238.—Wiring for an Office Building. Data on outlets and 

lighting feeders. 

(Mr. C. E. Knox, Consulting Engineer.) 


























































































































































































































340 


EXAMPLES OF WIRING SYSTEMS 


[CHAP. 21 


and enclosed fuses. Panels on the same floor, supplied from 
feeders A and C are both located in the same cabinet. Each 


MOTOR CIRCUITS 


NUMBER 

SERVICES 

FOR 

SUPPLYING 

CURRENT 

TO 

HP.SUPPLIED 

LENGTH IN FT. 

I ONE WAY) 

CURRENT IN AMP. 

LiJ 

02. 

£ 

Lu 

o 

LJ 

M 

lO 

MINIMUM INSIDE 
DIAM.OE CONDUIT 

ALLOWED 

/ 

loitetErh. Fan 

4 

250 

15 

No. 8 

/" 

z 

AirSupptv fan 

U'/z 

35 

45 

No.4 


3 

Exhaust Fan 

U'/z 

40 

45 

No. 4 

t>4 

A 

Vent Motor 

'/z 

too 

2 

No. 14 

Vz" 

5 

»• >•> 

Vz 

80 

2 

No./4 

Vz 

3 

Organ Motor 

/ 

70 

4 

NoJ4 

Vz" 

7 

Vent. Motor 

'/z 

125 

2 

No./4 

5z" 

8 

99 m 

'/Z 

125 

2 

No 14 

Vz" 

9 

tr /? 

Vz 

no 

2 

No. 14 

Vz" 

10 

n ft 

'/z 

no 

2 

No. 14 

5z" 

// 

Orqan Motor 

2 

180 

8 

No./O 

W 

/2 

Vent Motor 

/ 

!80 

4 

No 14 

'/z" 

13 

Fan Motor 

!2'/z 

25 

50 

No . 3 

r/4 

fa 

9i *t 

I 

no 

4 

No./4 

'/z' 

15 

| 

r* 

10 

30 

40 

No. 4 

r/4 

15 

*9 >1 

10 

35 

40 

No.4 

W 

n 

Fire Pump Motor 

to 

30 

40 

No. 4 

154 

18 

99 99 >• 

10 

35 

40 

No.4 

r/4 

19 

Elevator Motor 

40 

25 

150 

No4/o 

2" 

20 

99 n 

35 

25 

140 

No.4'0 

2" 

2/ 

>9 ** 

35 

25 

/40 

No4M 

2" 

?Z 

99 91 

35 

25 

140 

No.4 / o 

2" 

23 

9* 99 

35 

25 

140 

No.4 / o 

2" 

25 

99 99 

35 

25 

140 

N04M 

2" 

25 

99 9f 

35 

25 

140 

No.4'0 

2" 


SUPPLY FEEDER 


NUM8ER 

ESTIMATED 
LOAD IN 
AMP. 

AT 140 
VOLTS 

NUMBER OF 
CONDUCTORS 

SIZE OF 
EACH 

CONDUCTOR 

NUMBER OF 
CONDUITS 

MINIMUM INSIDE 
DIAMETER OF 
CONDUIT ALLOWED 

ESTIMATED 
•LEN6TH IN FT. 

( ONE WAY ) 

/ 

?000 

6 

No 2.000,000 

6 

3/z‘ 

200' 

Z 

2000 

& 

No 2,000,000 

e 

3hz" 

140' 









LIGHTING MAINS 


NUMBER 

SUPPLIED BY 

BRANCH CIRlS j 

a 

LJ 3: 

— <c 

u 

P Q 
</) <c 

U_J O 

LENGTH in FT. 

(ONEWAY) 

LOSS IN VOLTS 

ON EACH SIDE 

SIZE OF WIRE 

MINIMUM INSIDE 

DIAMETER OF 

CONDUIT ALLOWED 

FEEDER 

CUT-OUT 

FLOOR 

MIDDLE 

CONDUCTOR 

OUTSIDE 

CONDUCTOR 

AS 

6 

A 

4th 

20 

so 

17 


No 4 

No 4 

l/z 

AO 

7 

A 

7lh 

16 

40 

IS 


No. 6 

No 6 

nv 

AH 

7 

A 

7th 

18 

4S 

75 


No 6 

No 6 

/V 

A? 

9 

A 

10th 

16 

40 

15 


No 6 

No 6 

r/4 

All 

9 

A 

10th 

16 

40 

15 


No 6 

No 0 

1/4 

A12 

10 

A 

13th 

16 

40 

15 


No 6 

No 6 

1 / 4 " 

A! A 

10 

A 

/3th 

16 

40 

15 


No 6 

No 6 

1/4 

A IS 

12 

A 

16th 

16 

40 

15 


No 6 

No (? 

/V 

AH 

12 

A 

16th 

18 

4S 

15 


No 6 

NO 6 

1 / 4 • 

BM 

17 

6 

4th 

20 

50 

15 


No. S 

No 5 

r/4 w 

f) 5 

17 

B 

4 th 

16 

40 

16 


No 6 

No 6 

l'/4 

66 

IS 

6 

1W 

14 

3S 

!5 


No. 6 

No 6 

1/4 

BS 

IH 

6 

7th 

16 

40 

15 


No.6 

No 6 

1 / 4 " 

69 

19 

B 

/Ott 

14 

35 

15 


No 6 

No 6 

1/4 

BH 

19 

B 

toth 

14 

35 

IS 


No. 6 

No 6 

/'/* 

6/2 

70 

B 

ixh 

14 

35 

!5 


No. 6 

NO 6 

1 / 4 ' 

6/4 

20 

6 

nth 

14 

35 

15 


No 6 

No 6 

/V' 

BIS 

2/ 

B 

FEFh. 

14 

35 

15 


No 6 

No 6 

ft" 

617 

21 

B 

16th 

14 

35 

15 


No 6 

No 6 

1/4 

Co 

8 

C 

8th 

10 

2S 

IS 


No 8 

No 8 

7* 

C 7 

S 

C 

8th 

16 

40 

15 


No 6 

No. 6 

1/4 

C 9 

8 

C 

8th 

12 

30 

IS 


No 8 

No 8 

/* 

no 

8 

c 

8th 

6 

If 

75 


No./O 

No 10 

/* 

cn 

n 

c 

nth 

6 

/5 

IS 


No. 10 

No 10 

~T' 

C12 

// 

c 

Fth 

16 

40 

15 


No 6 

No 6 

r/E 

C!4 

// 

c 

nth 

12 

30 

15 


No 8 

No 8 

/' 

CIS 

// 

c 

nth 

6 

IS 

IS 


No./O 

No !0 

7" 

cn 

// 

c 

16th 

6 

15 

IS 


No 10 

No 10 

7" 

A 2 

3 

A 

6rH 

40 

100 

28 


No 0 

No 0 

2" 













POWER FEEDERS 


NUMBER. 

TERMINATES 

AT 

HP SUPPLIED 

ESTIMATED 
LOAD IN AMR 

h- — 

M- LJ 

J 

LOSS IN VOLTS 
ON EACH SIDE 

Size 

OF 

WIRE 

1. 1 O 
OL- ^ 

5°i 

~ at Ej 

3 uj <c 

HI 

IQ° 

73 

[. Roof 

75 

300 

4/5 


No 500.000 

3' 

24, 

D Roof 

105 

420 

440 


600 000 

3" 

25 

Doth Floor 

7V 

280 

260 


400 000 

3' 

26 

A Me 2 id 

34 

736 

240 


00 

2 * 

27 

C Basemen f 

738 

54 

135 


2 

/>2 

78 

Lift Motor 

123 

50 

725 


2 

/>; 

29 

House Pump 

20 

80 

45 


2 

n; 

30 

fire Pump 

20 

80 

96 


2 


37 

Orqan Motor 

70 

40 

700 


4 

fa' 

32 

Vacuum Clean 



55 


4 

fa 

33 

Sump Pump 



90 


8 

/’ 


Fig. 239.—Wiring for an Office Building. Data on feeders and 

mains. 

(Mr. C. E. Knox, Consulting Engineer.) 

cabinet has a compartment for push-button switches, which 
control the corridor lights supplied by feeders C. The cabinets 
are steel, with steel door and trim finished to match the decora- 











































































































































































PAR. 341 ] 


INDUSTRIAL LIGHTING 


341 


tions of the corridor. On the office floors there are about 16 
branch lighting circuits for each of the two panel boards supply¬ 
ing the floor. The power panel boards (Fig. 239), have from 
two to four double-pole, fused switches for supplying the indi¬ 
vidual motors. 

341. Arrangement of Branch Circuits. The lighting circuits 
each supply from three to six outlets (Fig. 236). Plug recep¬ 
tacles located in the wall about 18 in. above the floor are pro¬ 
vided in most of the rooms. These are supplied by separate 
branch circuits. The lamps in the ordinary offices are con¬ 
trolled by pendant switches attached to the lighting fixture. 

342. Lighting System. The ordinary offices are lighted by 
the direct system, using fixtures with two or three arms and 
translucent glass reflectors (“ Opalux ”). Each arm carries 
a 40- or 60-watt tungsten lamp. The lighting of the lower floors, 
which are used for sales, exhibition and executive offices, is by 
the semi-indirect system. 

343. Auxiliary Circuits. An idea of the various kinds of 
service provided can be obtained from Fig. 238, which gives the 
list of outlets. Two combined fire-alarm and watchman’s 
stations are located on each floor in the corridors. 

Installation in a Collar Factory 

The arrangement of circuits on one floor of a nine-story build¬ 
ing is shown in Fig. 240. 

344. Lighting. Energy for lighting is supplied at 125 volts 
from a four-wire, three-phase system. The lamps are con¬ 
nected between the outside wires and the neutral. Each light¬ 
ing feeder is made up of three wires, one being connected to the 
neutral and the other two across one of the phases. The feeders 
are distributed on the three phases to balance the load. The 
spacing of the units is rather wide—about 22 by 18 ft.—because 
the floor shown is used for storage and rough manufacturing. 
Exit lamps and emergency lamps on circuits separate from the 
regular supply are provided. The lamps are supplied from five 
panel boards on each floor and are controlled by switches on 
these panels. All wiring is in conduit. 

345. Power. The motors are connected across the three 
phases and hence operate at about 217 volts. Separate feeders 
and panel boards are provided for the power supply. 


342 


EXAMPLES OF WIRING SYSTEMS 


[chap. 21 


Lighting Installation for a Machine Shop 

346. In I'ig. 241 is shown a portion of a floor in a light machine 
shop, with a low ceiling (about 13 ft.). The work is on small 



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bfi 

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oi 

s-, 

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Ph 


parts and, therefore, a close spacing of the units—about 9X9 ft.—• 
is required. Emergency lighting and exit lamps are provided. 
4 he units are equipped with deep bowl, enamel-steel reflectors, 
supported from No. 14 reinforced cord. Because of this 
















































































































































PAR. 346 ] INDUSTRIAL LIGHTING 



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343 


























































































































344 


EXAMPLES OF WIRING SYSTEMS [CHAP. 21 



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arrangement, 1320 
watts can be allowed 
on one circuit. Plug 
outlets are provided, 
on circuits separate 
from the lamps, but 
using the same con¬ 
duits. 

Lighting Instal¬ 
lation for A 
Railroad Repair 
Shop 

347. An arrange¬ 
ment for a shop with 
relatively high ceil¬ 
ings is shown in Fig. 
242. The lamps are 
supplied on a 220- 
110 volt, three-wire, 
a.c. system. This 
is furnished by 
transformers step¬ 
ping down from 440 
volts, which is the 
voltage used for the 
motors. The mains 
which have slow- 
burning insulation 
are run open on 
insulators located 
on the roof trusses 
(Fig. 242). Taps 
are taken from these 
mains for the light¬ 
ing units, each of 
which is fused, as 
shown in the illus¬ 
tration. A special 
feature is the use 
of angle steel re- 















































































par. 347 ] 


POWER INSTALLATIONS 


345 



Fig. 243. —Motor Layout for Group Drive. 

(Westinghouse, Church, Kerr & Co.)' 































































































































































































































346 EXAMPLES OF WIRING SYSTEMS [CHAP. 21 

flectors near the walls better to distribute the light and give 
proper lighting on the benches near the walls. Plug outlets 
are provided, but the overhead lighting is intended to be 
sufficient for the usual requirements. The mains are separated 
about every 75 ft. by section breaks, and these sections are 

controlled from panels located on 
the columns at convenient intervals. 


Power Installation for Machine 
Shops 

348. Group Drive. The instal¬ 
lation shown in Fig. 243 illus¬ 
trates one method of wiring motors 
where group drive is used. A two- 
phase, 220-volt system is employed. 
The mains are run open on cleats 
and are located near the ceiling. 
Slow-burning insulation is used. 
Mains are run the length of the 
shop, at either side and the branch 
circuits are tapped off at convenient 
intervals. No panel boards are 
used, since the motors are too 
scattered to make this arrangement 
feasible. In Fig. 244 is shown the 
method of installing the auto-starters 
and the starting and running fuses. 
The branch circuits to the motors 
are in conduit. 

349. Individual Drive. The plan 

shown in Fig. 245 is typical of a 
machine shop doing fairly heavy work. This is part of a 
locomotive repair shop. Most of the motors are operated on a 
440-volt, three-phase, 60-cycle system, but a few are supplied 
with 230 volts, direct current. The latter system is used only 
for variable-speed tools and a few of the cranes. Most of the 
cranes, including one of 150-ton capacity, are operated from the 
a.c. supply. The d.c. cranes were taken from another shop. 
All the new crane installation is alternating current. The 
amount of the d.c. load is kept as small as possible to reduce the 



Fig. 244. — Arrangement 
of Motor Branch Cir¬ 
cuits. Group Drive. 

This shows method of 
mounting auto-starter and 
fuses for the motors shown in 
Fig. 243. (Westinghouse, 
Church, Kerr & Co.) 











































From Generator Panel 


par. 349 ] 


POWER INSTALLATIONS 


347 


*3 .2 



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348 


EXAMPLES OF WIRING SYSTEMS [CHAP. 21 


size of the motor-generator set required for this purpose. It 
will be noted that, in addition to the individual-drive motors, 
there are two operating group drives (Groups 6 and 7). The 
mains, which are not shown, are run open, near the ceiling along 
each of the columns. These mains consist of wire with slow- 
burning insulation, supported on porcelain cleats and held tight 
by strain insulators at each end. Taps from the mains supply 
panel boards located on the columns. These taps are in con¬ 
duit. The panel boards contain a fused main switch and fuses 
for each branch circuit. The branch wiring is all in conduit. 
Motors up to 5 hp. capacity are started by a two-throw switch 
which cuts out the running fuses when it is in the starting posi¬ 
tion. Large motors have auto-starters and running fuses or 
overload relays. 

Lighting Installation for a Hotel 

350. Fig. 246 shows a portion of one floor of a high-class 
hotel and illustrates the arrangement for the guests’ rooms. 
The lighting is supplied by a 240-120 volt, three-wire, d.c. 
system. All wiring is in rigid iron conduit concealed in the 
floors and walls. Pan6l boards on each floor supply the branch 
circuits. The panel boards contain a tliree-pole, fused main 
switch. Each branch circuit is provided with fuses and a knife 
switch. The lighting feeders are so designed that the drop 
between service switchboard and the furthest panel board is 
not more than 3.5 volts on either side of the neutral with all the 
lights burning. The corridor lamps are placed on two circuits, 
which are staggered, so that half of the lamps can be extin¬ 
guished late at night. Each bedroom is provided with a wall 
receptacle which can be used for a table light, etc. Wall 
brackets are located on each side of the dresser. General 
illumination of the room is provided by a ceiling outlet with a 
semi-indirect fixture. 

Lighting Installation for a Residence 

361. The plans shown in Figs. 247 and 248 illustrate a lighting 
installation for a moderate-sized residence. The service enters 
underground with the meter, service switch and cutout in the 
basement. From this point, the circuit runs to the panel board 
on the first floor. All wiring is in rigid conduit. In all of the 


POWER INSTALLATIONS 


349 


PAR. 351 ] 



—O wan “ i “ 

(^) Ceiling “ more than 1 lighs 

—Wall ... “ 

-£> “ Base riug 

[ Floor Plug 

-X D.P. Push Switch iV'above floor 


Fig. 246. —Lighting Installation for a Hotel. Guests’ rooms. 

(Westinghouse, Church, Kerr & Co.) 













































































































































































































































350 


EXAMPLES OF WIRING SYSTEMS [CHAP. 21 


□ a 

□ 


□ □ □ □ 

□ 



Fig. 247.—Lighting Installation for a Residence. First-floor plan. 







































































































































































PAR. 351 ] 


POWER INSTALLATIONS 


351 


large rooms on the first floor, the lamps are controlled by push¬ 
button switches. Plug outlets are provided in the living-room 
for reading lamps, vacuum cleaner, luminous heaters, etc. The 
plug outlet in the dining-room is for various cooking devices. 
The lamp in the upper hall is controlled by three-way switches 



Fig. 248.—Lighting Installation for a Residence. Second-floor 

plan. 


located on the first and second floors so that the lamp can be 
lighted or extinguished at either place. Each chamber is pro¬ 
vided with a ceiling outlet over the dresser. Plug outlets are 
supplied to accommodate a table lamp, luminous heater, and 
similar devices. 













































































































1 . . 














































• . 




































































APPENDIX A 

The paragraph number after each heading refers to the place in 
the text where a further description of the table may be found. 

TABLE 1. (Par. 22) 

Data on Metallized-filament (Gem) Lamps 


Rated 

Watts. 

Candle- 

power.* 

Rating, 

Total 

Lumens 

Rated 

Life.t 

Amperes 

Watts 

per 

Candle.* 

Output, 

Lumens. 

per 

Watt. 

at 120 
Volts. 

at 240 
Volts. 

20 

5.0 

4.0 

52.0 

2.60 

1000 

0.167 

0.083 

30 

10.0 

3.0 

103.8 

3.46 

1000 

0.250 

0.125 

40 

15.6 

2.56 

162.0 

4.05 

600 

0.333 

0.167 

50 

20.0 

2.50 

207.5 

4.15 

700 

0.416 

0.208 

60 

24.0 

2.50 

249.0 

4.15 

700 

0.500 

0.250 


* Mean horizontal candlepower. 
t See paragraph 8. 

Lamps of the above sizes may be obtained for voltages from 105 to 125. 


TABLE 2. (Par. 28) 


Data on Mazda B (Vacuum-type) Lamps for Constant Poten* 

tial Circuits * 


Rated 

Watts. 

Mean 

Spherical 

C.P. 

Rating 
Watts per 
Candle, f 

Total 

Output, 

Lumens. 

* 

Lumens 
per Watt. 

Ampei 

at 120 
Volts. 

EtES 

at 240 
Volts. 


10 

6 

0 

1 

.67 

75 

7 

50 

0 

083 



15 

10 

2 

1 

.47 

128 

8 

55 

0 

125 


o 

20 

14 

2 

1 

.41 

178 

8 

90 

0 

167 


K’* 

to 

25 

18 

5 

1 

.35 

234 

9 

30 

0 

208 


CM 1 

r—1 

40 

30 

3 

1 

.32 

380 

9 

50 

0 

333 


I 

»o 

50 

38 

2 

1 

31 

480 

9 

60 

0 

416 


t-H 

60 

47 

0 

1 

28 

590 

9 

80 

0 

500 



100 

82 

0 

1 

22 

1030 

10 

30 

0 

833 


CO 

r 25 

15 

2 

1 

65 

191 

7 

60 



0.104 

o 

40 

28 

2 

1 

42 

354 

8 

85 



0.167 

> 

O . 

60 

43 

2 

1 

39 

540 

9 

05 



0.250 

lO i 
CM 

100 

78 

7 

1 

27 

990 

9 

90 



0.417 

o 

150 

118 

0 

1 

27 

1480 

9 

90 



0.625 

(N 

250 

208 

0 

1 

20 

2620 

10 

50 



1.042 


* Manufacturers’ standard ratings, July 1, 1916. 
f Based on mean spherical candlepower. 

Rated life is 1000 hours for all lamps except the 10-watt size, which has a 


life of 1500 hours. 


353 
















































220-250 volts 105-125 volts 


354 


APPENDIX A 


TABLE 3. (Par. 31) 

Data on Mazda C (Gas-filled) Lamps for Constant Potential 

Circuits * 


Rated 

Watts. 


' 75 
100 
200 
300 
400 
500 
750 
.1000 

200 

300 

400 

500 

750 

1000 


Mean 

Spherical 

C.P. 

Rating. 

Watts 

per 

Candle, f 

Total 

Output, 

Lumens. 

Lumens 
per Watt. 

Ampi 

at 120 
Volts. 

2RES 

at 240 
Volts. 

69 

1.09 

865 

11.5 

0.625 


100 

1.00 

1,260 

12.6 

0.833 


222 

0.90 

2,800 

14.0 

1.67 


366 

0.82 

4,600 

15.3 

2.50 


488 

0.82 

6,150 

15.3 

3.33 


641 

0.78 

8,050 

16.1 

4.17 


1010 

0.74 

12,800 

17.0 

6.25 


1430 

0.70 

18,000 

18.0 

8.33 


200 

1.00 

2,520 

12.6 


0.833 

326 

0.92 

4,100 

13.7 

i 

1.25 

445 

0.90 

5,600 

14.0 


1.67 

588 

0.85 

7,400 

14.8 


2.08 

915 

0.82 

11,500 

15.3 


3.13 

1280 

0.78 

16,100 

16.1 


4.17 


* Manufacturers’ standard ratings, July 1, 1916. 
f Based on mean spherical candlepower. 

Rated life is 1000 hours for all sizes. 




























APPENDIX A 


355 


TABLE 4. (Par. 47) 

Performance of Typical Enclosed Carbon Arc Lamps 



Series. 

Multiple. 

D.C. 

A.C. 

A.C. 

D.C. 

D.C. 

D.C. 

A.C. 

A.C. 

Intensified 

Arc, 

D.C. 

Terminal voltage 

75 

77 

77 

110 

110 

220 

110 

110 

110 

Arc voltage. 

73 

72 

72 

80 

80 

150 

72 

72 

80 

Amperes. 

6.6 

6.6 

7.5 

5 

6.5 

3.25 

6 

7.5 

5 

Watts. 

495 

425 

480 

550 

715 

715 

430 

540 

550 

Power factor.. 


0 84 

0.84 




0.65 

0.65 


Electrode life,hrs. 

125 

125 

100 

150 

100 

150 

125 

100 

75 

M.L.H.C.P*. . . . 

479 

232 

291 

379 

559 

215 

276 

371 

414 

Watts per 




- 






M.L.H.C.P.*. 

1.03 

1.83 

1.65 

1.45 

1.28 

3.33 

1.56 

1.45 

1.33 

M.S.C.P. f . 

290 

144 

173 

215 

318 

160 

160 

215 

225 

Watts per 










M.S.C.P. f ... 

1.71 

2.95 

2.77 

2.56 

2.25 

4.44 

2.68 

2.51 

2.44 

Lumens. 

3650 

1810 

2175 

2700 

4000 

2010 

2010 

2700 

2830 

Lumens per watt 

7.37 

4.25 

4.53 

4.86 

5.58 

2.81 

4.67 

4.98 

5.14 

Reflector. 

Porcelain en- 

Porcelain 





ameled steel 







Inner globe. 

Clear 


Opal 


- 



Outer globe. 

Clear 


None 






* M.L.H.C.P. = mean lower hemispherical candlepower. 
f M.S.C.P. = mean spherical candlepower. 



































356 


APPENDIX A 


TABLE 5. (Par. 52) 

Performance of Typical Enclosed, Flame-arc Lamps 



Series. 

Multiple. 

a.c. 

A.C. 

A.C. 

D.C. 

D.C. 

Terminal voltage. 

55 

110 

110 

110 

110 

Arc voltage. 

49 

48 

48 

70 

70 

Amperes. 

10 

7.5 

7.5 

6.5 

6.5 

Watts. 

445 

500 

500 

715 

715 

Power factor. 

0.81 

0.61 

0.61 



Electrode life, hours.... 

100 

100 

100 

100 

100 

M.L.H.C.P.*. 

1400 

1490 

920 

1320 

1250 

Watts per M.L.H.C.P.*. 

0.32 

0.34 

0.54 

0.54 

0.57 

M.S.C.P. f. 

769 

850 

695 

797 

625 

Watts per M.S.C.P.f . .. 

0.58 

0.59 

0.72 

0.90 

1.14 

Lumens. 

9670 

10,680 

8740 

10,010 

7860 

Lumens per watt. 

21.70 

21.4 

17.44 

14.02 

11.0 

Reflector. 

None 

None 

None 

None 

Steel 

Inner globe. 

Clear 

Clear 

Clear 

Clear 

Clear 

Outer globe. 

Clear 

Clear 

Trans- 

Trans- 

None 




lucent 

lucent 



White-light carbons used. Yellow-light carbons givu about 25 per cent 
more light. 

* M.L.H.C.P. =raean lower hemispherical candlepower. 
t M.S.C.P. =mean spherical candlepower. 






























APPENDIX A 


357 


TABLE 6. (Par. 56) 

Performance of Typical Metallic-electrode, Series Arc 

Lamps 



Standard 

Electrodes. 

High Efficiency 
Electrodes. 

Pendant 

Type. 

Orna¬ 

mental 

Type. 

Pendant Type. 

Orna¬ 

mental 

Type. 

Terminal voltage. 

75-80 

75-80 

80 

75-80 

75-80 

75-80 

80 

Arc voltage. 

74-79 

74-79 

79 

74-79 

74-79 

74-79 

79 

Amperes. 

4 

6.6 

5 

4 

5 

6.6 

5 

. 

310 

510 

400 

310 

388 

510 

400 

Electrode life, hours. . . 

350 

125 


150 

120 



M.L.H.C.P.*. 

488 

1266 

510 

741 

1131 

1626 

510 

Watts per M.L.H.C.P.* 

0.63 

0.403 

0.78 

0.42 

0.34 

0.314 

0.52 

M.S.C.P.t. 

256 

668 

418 

425 

637 

948 

631 

Watts per M.S.C.P.f • • 

1.21 

0.76 

0.96 

0.73 

0.61 

0.54 

0.63 

Lumens. 

3220 

8400 

5250 

5340 

8000 

11,900 

7940 

Lumens per watt. 

10.40 

16.50 

13.1 

17.2 

20.6 

23.40 

19.8 


* M.L.H.C.P. =mean lower hemispherical candlepower. 
f M.S.C.P. =mean spherical candlepower. 

All pendant lamps have clear globes; ornamental lamp has translucent 
globe. 

Pendant lamps with standard electrodes have internal reflector; with high 
efficiency electrodes. Lamps have refractor. (See Fig. 9.) 




























358 


APPENDIX A 


TABLE 7. (Par. 60) 

Performance Data for Typical Cooper Hewitt Mercury 

Vapor Lamps * 


Type of Lamp. 

Current. 

Tube Material. 

Tube Length, Inches. 

Method of Lighting. 

Amperes. 

Volts. 

Average Watts. 

Initial Lower Hemispher¬ 

ical Candlepower. 

Initial Watts per Lower 

Hemispherical Candle- 

power. 

Initial Spherical Candle- 

power without Reflector. 

Initial Watts per Spher¬ 

ical Candlepower 
without Reflector. 

H 

D.C. 

Glass 

21 

Auto- 

tilting 

3.5 

100-125f 

192 

300 

0.64 

190 

1.01 

P 

D.C. 

Glass 

50 

Shifter 

3.5 

100-125 

385 

800 

0.48 

500 

0.77 

K 

D.C. 

Glass 

45 

Hand- 

tilting 

3.5 

100-125 

385 

700 

0.55 

440 

0.87 

L 

D.C. 

Glass 

35 

Shifter 

2.1 

100-125 

220 

400 

0.55 

250 

0.88 

Ft 

A.C. 

Glass 

50 

Shifter 

4.1 

100-125 

380 

800 

0.48 

500 

0.77 

EJ 

A.C. 

Glass 

35 

Shifter 

2 

100-125 

200 

400 

0.5 

250 

0.80 

Z 

D.C. 

Quartz 

4 

Auto- 

tilting 

3.3 

200-240 

726 

2400 

0.3 

1500 

0.48 


* Data supplied by Cooper Hewitt Electric Co. 
t Two in series on this voltage, 
t Power factor 87 per cent. 

All above are low-pressure types except Type “ Z.” 































APPENDIX A 


359 


TABLE 8. (Par. 118) 

Illumination Intensities for Commercial Lighting 

Explanation. —Column 2 indicates method of illumination most suitable. 
G = general (uniform) illumination. L=localized lighting, L-G = group light¬ 
ing (non-uniform general illumination), G&L=combined local and general 
lighting. Column 3 gives the system of illumination most commonly used. 
Where two are given, the first is usually employed. D = direct system, SI = 
semi-indirect, I =indirect. Column 4 gives average value of foot-candles 
intensity or the lumens per square foot required. (See foot-note at end of 
table.) 


1 

Class of Service. 

2 

Meth. 

of 

Ilium. 

3 

System 

of 

Ilium. 

4 

Foot- 

candles. 

5 

Reflector Equipment. 

Armory. 

G 

\ D 

2.0- 3.0 

Metal or prismatic glass 

Assembly room. 

G 

D, SI, I 

0.5- 1.5 

Prismatic or white glass 

Auditorium. 

Automobile show- 

G 

D, SI, I 

1.0- 3.0 

Prismatic or white glass 

room. 

G 

SI, D\ 

3.0- 6.0 

Prismatic or white glass 

Ball room. 

G 

D, SI 

2.0- 5.0 

White glass or bare frosted 
lamps 

Bank (general). 

G 

SI, D 

2.0- 3.0 

White glass 

Bank (desk work) . . 

G & L 

SI, D 

4.0- 8.0 

Same general lighting with 
desk lamps 

Barber shop. 

G 

SI, I, D 

3.0- 5.0 

White glass or prismatic 
Mirror refl. for I 

Billiard room (gen.). 

G 

D 

0.8- 1.5 

Prismatic or white glass 

Billiard room (table). 

L 

D 

6.0-10.0 

Prismatic glass or metal 

Bowling (alley). 

L-G 

D 

1.5 

Eight metal, angle refl. with 
40-watt lamps 

Bowling (pins)...... 

L-G 

D 

4.0- 6.5 

Angle refl. in front of pins, 
40-watt lamp 

Caf6 (general only) . 

G 

SI, D 

2.0- 4.0 

White glass or prismatic 

Caf6 (with table 
lamps) 

G & L 

SI 

1.0- 2.0 

White glass or prismatic, 
opaque shades on table 
lamps 

Card room. 

G 

D 

2.0- 3.5 

Prismatic or white glass 

Church. 

G 

SI, I, D 

1.0- 3.0 

White glass. Mirror refl. 
for I 

Corridors. 

G 

D, SI 

0.5- 1.5 

White glass or prismatic 
enclosing globes 

Court room. 

G 

D 

2.0- 4.0 

White glass or prismatic 

Desk. 

L 

D 

4.0- 6.0 

Metal reflector, enclosing 
lamp 

Garage... 

G & L 

D 

1.0- 3.0 

Metal or prismatic. Hand 
lamps 






























360 


APPENDIX A 


TABLE 8 —Continued 


1 

Class of Service. 

2 

Meth. 

of 

Ilium. 

3 

System 

of 

Ilium. 

4 

Foot- 

candles 

5 

Reflector Equipment. 

Gymnasium. 

Hospital: 

G 

D 

2.5- 4.0 

Metal or prismatic. Hand 
lamps 

Ward (dim). 

G 

SI, I 

0.2- 0.3 

| White glass or mirror 

Ward (bright).. . . 

G 

SI, I 

1.0- 2.0 

•j Dim light used dur- 
[ ing sleeping period 

Operating table. . 

L-G 

D 

25 or more 

White glass angle reflectors 

Hotel: 





Bedroom. 

G & L 

SI, D 

1.5-3.0 

White glass or prismatic. 
Wall brackets near dres¬ 
ser. Table lamp 

Dining room. 

G 

SI, D 

2.0-4.0 

White glass or prismatic 

Dining room. 

G & L 

SI 

1.0-2.0 

White glass or prismatic. 
Table lamps with opaque 
shades 

Lobby. 

G 

SI, D 

2.0-4.0 

White glass or prismatic 

Library: 





Stack room. 

L-G 

D 

1.5-2.0 

Prismatic or white glass 

Reading room.. . . 

G 

SI 

3.0-4.0 

White glass 

Reading room.. . . 

G & L 

SI, D 

0 

1.0-1.5 

White glass or prismatic. 
Table lamps with opaque 
reflectors 

Office: 





Small (private).. . 

G 

SI, D 

1.0-2.0 

White glass or prismatic, 
with desk lamp 

Large. 

G 

SI, D 

4.0-8.0 

White glass or prismatic 

Large. 

G & L 

SI, D 

1.5-2.0 

White glass or prismatic, 
with 40-watt bowl frosted 
lamp and metal refl. for 
desk 

Public spaces. 

.... 


0.5-1.5 

Glass or metal 

Reading, clear print. 

.... 


1.0-1.5 


Reading, newspaper. 

.... 


2.0-3.0 


Residence. 





Hall. 

G 

SI, D 

0.7-1.0 

White glass 

Parlor or living 





room. 

G 

SI, D 

1.0-3.0 

1 White glass. May be sup- 

Library . 

G 

SI, D 

2.0-4.0 

/ plemented by table lamp 

Dining room. 

G 

SI, D 

1.0-3.0 

White glass 

Kitchen. 

G 

D 

2.0-3.0 

White glass or prismatic. 

Bedroom.'. 

G & L 

SI, D 

1.0-3.0 

White glass. Wall brack¬ 
ets near dresser 

Storeroom. 

G 

D 

0.4-0.8 

Prismatic or bare lamp 

































APPENDIX A 


361 


TABLE 8 —Continued 


1 

Class of Service. 

2 

Meth. 

of 

Ilium. 

3 

System 

of 

Ilium. 

4 

Foot- 

candles. 

5 

Reflector Equipment. 

Restaurant. 




See hotel dining room 

Saloon. 

G 

D, SI 

2.0-4.0 

Prismatic or white glass 

School: 





Classroom. 

G 

SI, D 

3.5-5.0 

White glass or prismatic 

Laboratory. 

Store: 

G 

D, SI 

3.0-5.0 

White glass or prismatic 

Clothing. 

G 

SI, D 

4.0-7.0 

White glass or prismatic 

Dry goods. 

G 

SI, D 

4.0-7.0 

White glass or prismatic 

Furniture. 

G 

SI, D 

3.0-6.0 

White glass or prismatic 

Grocery. 

G 

D, SI 

2.0-4.0 

White glass or prismatic 

Hardware. 

G 

D 

3.0-5.0 

White glass or prismatic 

Jewelry. 

G, 

G & L 

D, SI 

4.0-6.0 

White glass or prismatic. 
With direct units over the 
counters 

Millinery. 

G 

SI, D 

4.0-6.0 

White glass or prismatic. 
(Color matching unit de¬ 
sirable) 

Shoe. 

G 

SI, D 

2.0-4.0 

White glass or prismatic 

Stationery. 

G 

SI, D 

2.0-4.0 

White glass or prismatic 

Tobacco. 

G 

D, SI 

2.0-4.0 

White glass or prismatic 

Show, window: 





High grade. 

G 

D 

15-30 

Glass or metal, angle reflec. 

Ordinary. 

G 

D 

10-20 

Glass or metal. Angle or 
symmetrical 

Small town. 

G 

D 

5-15 

Glass or metal 

Stable. 

G 

D 

0.8-1.0 

Glass or metal 

Theatre: 




White glass or prismatic 

Auditorium. 

G 

D 

1.0-3.5 

Moving picture.. . 

G 

D, SI 

0.2-0.5 

White glass or prismatic 
(during exhibition) 

Moving picture.. . 

G 

D, SI 

2.0-2.5 

White glass or prismatic 
(during intermission) 


Values of foot-candles for (G) are average over entire room, for (G & L) 
value is the illumination produced by G. Illumination on work in this case 
is higher due to L. Where L-G is specified, the value is illumination on work. 
The average value in this case will be somewhat lower. 

Col. 4 from data published by Holophane Works of General Electric Co. 

and National X-ray Reflector Go. 





































362 


APPENDIX A 


TABLE 9. (Par. 118) 


Illumination Intensities for Industrial Lighting * 


Explanation. —Column 2 indicates method of illumination most suitable. 
G = general (uniform) illumination, L =localized lighting, L-G =group light¬ 
ing (non-uniform general illumination), G & L = combined local and general 
lighting. 

Column 3 gives average value of foot-candles intensity or the lumens per 
square foot required. (See foot-note at end of table.) 

All illumination by the direct system unless otherwise noted. 


1 

2 

3 


Method 


Class of service. 

of 

J: OOt- 


Ilium. 


Bakeries. 

G 

2-3 

Bench work: 






3-5 for 

Single benches. 

L-G, G 


rough 

Single benches at wall. 

G, L-G 

< 

5-10 for 

Double benches. 

G, L- G 


fine 




work 

Book binding: 



Folding, assembling 



pasting. 

G 

2-3 

Cutting, punching, 



stitching .... 

G 

3-5 

Embossing. 

G 

4-6 

Candy factory. 

G 

2-4 

Canning factory. 

G, L-G 

1.0-2.5 

Carpenter shop. 



Clothing mfg.: 



Cutting, pressing. 

L-G 

5-10 

Machine sewing, dark 



goods. 

G & L 

10-15 

Machine sewing, light 



goods. 

G & L 

4-6 

Hand sewing, dark goods. 

G 

6-8 

Hand sewing, light goods. 

G 

4-6 

Inspecting. 

L-G 

6-10 


4 

Remarks. 


Prismatic or steel reflectors. 

40 to 100-watt lamps, spaced 
G to 10 feet 

60 to 150-watt lamps, spaced 
8 to 12 feet 


Steel or prismatic glass refl. 

Steel or prismatic glass refl. 
Steel or prismatic glass refl. 
Steel or prismatic glass refl. 

See bench work 
See wood-working 
Prismatic or steel. G of 1.0-1.5 
foot-candles 
See bench work 

15-watt lamp with metal refl. 
for each machine 

10-watt lamp with metal refl. 
for each machine 


See bench work 


Values in column 3 are for illumination on the work unless otherwise stated. 
* From data published in Handbook of Incandescent Lighting, General 
Electric Co. 



























APPENDIX A 


363 


TABLE 9 —Continued 


1 

Class of Service. 

2 

Method 

of 

Ilium. 

3 

Foot- 

candles. 

4 

Remarks. 

Cotton mill: 

Receiving and opening 



Steel reflectors 

bales. 

G 

0.8-1.5 

* 

Opening and lapping. . 

' « 

L-G 

1-2 

One 40-w. lamp at each end of 
machine 

Carding. 

L-G 

1.5-2.5 

One 40-w. lamp per machine 
staggered front and back. 

Drawing frame. 

L-G 

1.5-2.5 

Two 40-w. lamps per machine 

Roving frame. 

Ring spinning, spool- 

L-G 

2-3 

Two 40-w. lamps per machine 
over aisle 

ing, etc. 

L-G 

2-3 

Two 60-watt lamps over aisle, 
spaced every 100 spindles 

Slashing. 

L-G 

1-2 

Two 40-watt lamps per ma¬ 
chine over aisle 

Warpers. 

L-G, G 

2-3.5 

One 60-watt lamp over beam 
and one over rack 

Weaving, light goods.. 

L-G 

2-4 

One 60-w. for 4 mach. in aisle 

Weaving, dark goods.. 

L-G 

3-5 

One 100-w. lamp for 4 machines 
in aisle. 

Dyeing. 

L-G 

2-3 


Dyeing, inspecting.. . . 

L-G 

15-30 

Special color-matching unit 

Inspection. 

L-G 

5-10 

One 60-watt lamp over each 
table, with G 

Dairies and milk depots.. 

G, L-G 

1-4 

Enamel steel reflectors 

Drafting room. 

G. L-G 

6-12 

Indirect, semi-indirect or direct 
lighting may be used. Mir¬ 
ror reflectors for I, glass re¬ 
flectors for other systems 

Engraving. 

L 

10-15 

Steel reflectors 

Steel or prismatic glass refl. 

General with local ltg.. 

G & L 

1.0-2 

General, no local ltg. . 
Forge and blacksmith 

G 

3-6 


shop. 

Foundry. 

G, L-G 

2-4 

Enamel steel reflectors 

Enamel steel reflectors 

Moulding, machine . . . 

L-G 

1-3 

1 40 to 100-watt lamps with 

Moulding, floor. 

G & L 

1-2 

/ portable lamps 

Core making. 

L-G 

1-3 

See bench work 

Cupola, firing floor. . . 

G 

1-2 


Cleaning. 

G 

1-2 




































364 


APPENDIX A 


TABLE 9 —Continued 


1 

Class of Service. 

2 

Method 

of 

Ilium. 

3 

Foot- 

candles. 

4 

Remarks. 

Glove factory. 



Prismatic or steel ref’l. Ap¬ 
proximately white light req’rd 

Sorting. 

L-G 

6-10 

See bench work 

Cutting, trimming, in- 




specting. 

L-G 

5-6 

See bench work 

Sewing. 

Knitting mill. 

G & L 

8-12 

See clothing mfg. 

Steel reflectors 

Ordinary knitting. . . . 

L-G 

2-4 

One 60-watt lamp for 4 mach. 

Flat knitters. 

L-G 

3-5 

60-watt lamps in aisle, 6 to 8- 
foot centres 

Looping, seaming and f 

G 

4-6 

For general work 

finishing. \ 

L 


For fine work. 20-watt lamp 
for each machine 

Napper machines. 

L-G 

2-3 

40-watt lamp each machine 
over front rolls. 

Inspecting. 

L-G 

3-6 


Laundry. 

Machine shops. 

G 

3-5 

Steel or prismatic reflectors 
Enamel steel reflectors 

Bench work, fine. 

L-G, L 

5-10 

See bench work 

Bench work, rough. . . 

L-G 

3-5 


Machine tools, fine work. 

G, L-G 

5-8 


Machine tools, coarse 




work. 

G, L-G 

3-5 


Buffing and grinding. . 

L-G 

2-4 


Assembling and erect’g 

L-G 

1-3 


Inspecting. 

Offices. 

G 

4-7 

Supplemented by local units as 
needed. 

See commercial lighting, Tab. 8 

Packing and shipping. . . 

. 


Steel or prismatic reflectors 

Fine work. 

G, L-G 

3-5 

See bench work 

Ordinary work. 

G 

1.5-2.5 

See bench work 

Painting and finishing.. . 

. 


Steel reflectors 

Automobile, furniture, 




etc. Fine work. 

G 

4-8 


Ditto, coarse work, .'s . 

G 

2-4 


Paint and ink works.... 
Pottery. 

G 

2-4 

Steel reflectors 

Steel reflectors 

Grinding. 

G 

1-2 

Moulding, cleaning 




and trimming. 

L-G, G 

2-4 


Inspecting. 

G, L-G 

4-6 











































APPENDIX A 


365 


TABLE 9 —Continued 


1 

Class of Service. 

2 

Method 

of 

Ilium. 

3 

Foot- 

candles. 

4 

Remarks. 

Priwfir house . 



Steel or prismatic reflectors 

Engine room. 

G & L 

2-3 

Boiler room. 

G & L 

0.8-1.5 





Prismatic or steel reflectors 

Linotype and monotype 

G & L 

5-10 

2 foot-candles for G with 25- 




watt local lamps 

Typesetting. 

G & L 

6-8 

2 f-c. for G with 40-w. lamps 




every 4 ft. of type tray 

Matrixing and casting. 

G 

2-4 


Presses. 

L-G 

3-5 

One or more 60-watt lamps. 

Cutting and folding.. . 

G, L-G 

2-4 





Steel reflectors 

Punching, stamping.. . 

G 

2-5 

Dome or angle reflectors 

Cutting, shearing and 




spinning. 

G, L-G 

3-5 


Polishing and finishing. 

G, L-G 

2-4 





See packing and shipping 




Steel or prismatic refl., with G 




of 1 to 1.5 foot-candles 

Inspecting and sorting 

L-G 

7-9 

60-watt lamps each position 

Cutting. 

L-G, G 

5-7 

40-watt lamps on 5-ft. centres 

Stitching (machine).. . 

G & L 

6-8 

25-watt lamp in angle refl. 

Lasting and welting.. . 

L, L-G 

4-6 

40-watt lamp each machine 




Steel reflectors 

Winding frames.. 

L-G 

2-4 

Three 60-watt lamps per frame 

Throwing frames. 

L-G 

2-4 

60-watt lamps in aisle, spaced 




8 to 10-ft. centres 

Quilling. 

L-G 

3-5 

60-watt lamps in aisle spaced 




5 to 7 ft. centres 

Warping. 

L-G 

3-5 

A 60-watt lamp over creel, reed 




and reel 

Weaving. 

L-G 

4-6 

60-watt lamp over loom, 60- 




watt lamp in rear alley 

Dyeing. 

L-G 

2-3 


Dyeing, inspection.. . . 

L 

15-40 

Special color-matching unit 

Finishing. 

L & G 

3-5 

60-watt lamp over each table 




Steel reflectors 

Unloading yards, open 




hearth floors, cast 




houses, ore yards. . . 

G 

0.1-0.3 

Radial wave or dome refl. 

Loading yards. 

G 

0.3-0.5 

Radial wave or dome refl. 














































366 


APPENDIX A 


TABLE 9 —Continued 


1 

Class of Service. 

2 

Method 

of 

Ilium. 

3 

Foot- 

candles. 

4 

Remarks 

Steel Works: 




Rolling mills. 

G 

1-3 

Dome or angle refl. 

Wire drawing. 

G 

2-8 

High value for fine work 

Stock rooms(open rooms) 



Steel or prismatic refl. 

Rough material. 

G 

0.5-1.0 

Fine material. 

G 

0.5-3 0 


Warehouses. 

G 

0.5-1.0 

Steel reflectors 

Woodworking. 



Steel reflectors 

Rough work. 

G 

2-4 

Fine work. 

G 

4-6 


Woolen mill. 



Steel reflectors 

Receiving and picking. 

G 

2-4 


Washing and combing. 

G 

3-4 


Carding. 

L-G 

2-3 

60-watt lamp in aisle, every 




two machines 

Twisting. 

L-G 

2-3 

40-watt lamp in aisle, spaced 




on 7 to 10 ft. centres 

Dyeing. 

L-G 

2-3 

60-watt lamp every other tank 

Dyeing, inspecting.. . . 

L 

15-35 

Special color matching unit 

Warping. 

L-G 

3-5 

60-watt lamp over reel and reed 





60-w. lamp over lay, 40-w. lamp 





in each aisle. For goods 

Weaving light goods. . 

L-G 

4-6 


wider than 36 in. use two 40- 

Weaving, dark goods.. 

L-G 

6-8 


w. lamps over loom and one 





60-w. in each aisle. For dark 





goods use next larger size lamp 

Perching. 

L-G 

8-15 

100-w. lamp over each frame 

Perching, dark goods.. 

L-G 

10-20 

150-w. lamp over each frame 






























APPENDIX A 


367 


TABLE 10. (Par. 120) 

Utilization Efficiencies for Tungsten Lamps * 


These constants are the percentage of the total light flux produced by a 
tungsten lamp which is useful in producing illumination on a horizontal plane. 


Reflector 

Color of Ceiling, f 

Light. 

Light. 

Light. 

Me¬ 

dium. 

Me. 

dium. 

Dark. 

Equipment. 

Color of Walls, f 

Light. 

Me¬ 

dium. 

Dark. 

Me¬ 

dium. 

Dark. 

Dark. 

Prismatic glass 
finish) t. 

reflectors (velvet 

0.53 

0.50 

0.45 

0.45 

0.42 

0.38 

Prismatic glass enclosing globes. . 

0.51 

0.47 

0.44. 

0.43 

0.38 

0.35 

Translucent glass reflectors. 

0.49 

0.44 

0.41 

0.40 

0.37 

0.34 

Steel reflectors (enamel or alumi¬ 
num finish). 

0.48 

0.46 

0.44 

0.45 

0.44 

0.44 

Mirror reflectors (direct system). 

0.60 

0.53 

0.48 

0.48 

0.45 

0.40 

Translucent glass enclosing globes 

0.34 

0.31 

0.28 

0.26 

0.24 

0.19 

Semi-indirect bowls. 

0.40 

0.37 

0.33 

0.25 

0.20 


Indirect—steel reflectors. 

0.31 

0.28 

0.25 

0.18 

0.15 


Indirect—mirror reflectors. 

0.35 

0.33 

0.30 

0.20 

0.17 


Bare lamp. 


0.41 

0.35 

0.30 

0.30 

0.25 

0.21 


* Based on data published by Nat. Elec. Light Ass’n, 1916, and Lighting 
Handbook published by Holophane Works, of General Elec. Co., 1915. 
t For classification of terms—light, medium and dark, see Table 11. 
t For clear reflectors or for velvet finish with gas-filled lamps, use values for 
mirror reflectors, direct system. 

Note. The values given above apply to rooms having 200 to 1000 sq.ft, 
area. For rooms smaller than 200 sq.ft., the effect of reflection from the walls 
becomes important and the values should be reduced 10 to 40 per cent. The 
greatest reduction should be made for dark walls or very small rooms, since in 
these cases considerable light strikes the walls and never reaches the working 
plane. For rooms larger than 1000 sq.ft, the values can be increased some¬ 
what (not more than 15 per cent) particularly for medium or dark walls. Where 
the area to be illuminated forms a part of a larger area, and no appreciable 
light reaches the part under consideration from the remainder of the room, 
the values for dark walls should be used. If the ceiling of the room is made 
up of skylights, use the values for dark ceiling. 
































368 


APPENDIX A 


TABLE 11. (Par. 120) 


Color Classification of Walls and Ceilings 


Light. 

Medium. 

Dark. 

White 

Medium light buff 

Light brown 

Cream 

Faint pink 

Light blue 

Very light buff 

Light green 

Tan 

Light orange yellow 

Light gray 

Bluish white 

Pale gray 

Medium green 
Light red 


TABLE 12. (Par. 120) 

Power Required to Produce One Foot-candle Illumination 

Tungsten lamps,* uniform lighting. 





Watts per Square 

Foot 


Reflector 

Color of Ceiling, f 

Light. 

Light. 

Light. 

Me¬ 

dium. 

Me¬ 

dium. 

Dark. 

Equipment. 

■ Color of Walls, f 

Light. 

Me¬ 

dium. 

Dark. 

Me¬ 

dium. 

Dark. 

Dark. 

Prismatic glass reflectors (velvet 
finish) X . 

0.193 

0.204 

0.227 

0.227 

0.243 

0.269 

Prismatic glass enclosing globes. . 

0.200 

0.217 

0.232 

0.237 

0.269 

0.291 

Translucent glass reflectors. 

0.208 

0.232 

0.249 

0.245 

0.276 

0.300 

Steel reflectors (enamel or alumi¬ 
num finish). 

0.213 

0.222 

0.232 

0.227 

0.232 

0.232 

Mirror reflectors (direct system). 

0.170 

0.193 

0.213 

0.213 

0.227 

0.255 

Translucent glass enclosing globes 

0.300 

0.329 

0.365 

0.392 

0.425 

0.537 

Semi-indirect bowls. 

0.255 

0.276 

0.310 

0.410 

0.510 


Indirect—steel reflectors. 

0.329 

0.365 

0.408 

0.567 

0.680 


Indirect—mirror reflectors. 

0.291 

0.309 

0.340 

0.510 

0.600 


Bare lamp.. . . 


0.249 

0.291 

0.340 

0.340 

0.408 

0.485 


* Values are correct for 60-watt vacuum-type lamps (Mazda B) operating 
at 1.28 w. per m.s.cp. and giving 9.8 lumens per watt. Smaller lamps have 
slightly higher and larger lamps slightly lower values. For gas-filled lamps 
(Mazda C) multiply by 0.64. This is correct for 300 or 400-w. lamps. Other 
sizes give slightly higher or lower values. Values are based on the utilization 
efficiency given in Table 10 and are for rooms of 200 to 1000 sq.ft, area. The 
corrections are opposite from those given in Table 10, i.e., the values should be 
increased for smaller rooms and decreased for larger rooms. 

t For classification of terms—light, medium and dark, see Table 11. 

X For clear reflectors or for velvet finish with gas-filled lamps use values for 
mirror reflectors, direct system. 












































APPENDIX A 


369 


TABLE 13. (Par. 120) 

Power Required for Tungsten Lighting Systems 

Watts per square foot for uniform illumination 


Service. 

Foot- 

can¬ 

dles. 

With Vacuum 
Type Lamps.* 

With Gas-filled 
Lamps.* 

Direct. 

Semi- 

In¬ 

direct. 

In¬ 

direct. 

Direct. 

Semi- 

In- 

direct. 

In¬ 

direct. 

Auditorium. 

2 


0 

47t 

0 

69 

0 

77 

0 

30f 

0 

44 

0 

.50 

Corridors and halls. 

1 


0 

24t 

0 

33 

. 

. . 

0 

15f 

0 

21 



Drafting rooms. 

10 


2 

40 

3 

30 

4 

00 

1 

54 

2 

10 

2 

.56 

Engraving. 

12 


2 

80 





1 

80 





Factory, coarse work. 

2 


0 

59 





0 

38 





Factory, fine work. 

5 


1 

33 





0 

85f 





Factory, inspecting. 

10 


2 

66 





1 

70 % 





Gymnasium. 

3 


0 

72f 





0 

46f 





Hospital (ward). 

1 

5 

. 

. . 

0 

50 

0 

56 


. . 

0 

32 

0 

36 

Hotel (bedroom). 

2 

5 

0 

64 

0 

83 

0 

93 

. 

• . 

0 

53 

0 

60 

Library (reading room).. . 

4 

0 

1 

00 

1 

40 

1 

55 

0 

64f 

0 

90 

1 

00 

Office, general. 

4 

0 

1 

00 

1 

40 

1 

55 

0 

641 

0 

90 

1 

00 

Office, special. 

8 

0 

2 

00 

2 

80 

3 

10 

1 

28f 

1 

80 

2 

00 

Packing and shipping 















coarse work. 

2 


0 

51 

. 

. . 

. 

. . 

0 

33 





Packing and shipping 















fine work. 

4 


1 

00 

. 

. • 



0 

64 





Power house, engine room. 

2. 

5 

0 

69 

. 

. • 

. 

. . 

0 

44 





Power house, boiler room.. 

1 

0 

0 

30 

. 

. . 

. 

. . 

0 

19 





Residence. 

2 

0 

0 

48 

0 

64 

0 

74 


. . 

0 

41 

0 

.48 

Restaurant. 

3. 

0 

0 

72f 

1 

00 

1 

11 

0 

46+ 

0 

64 

0 

.71 

School. 

4. 

0 

1 

00 

1 

40 

1 

55 

0 

641 

0 

90 

1 

.00 

Store. 

5. 

0 

1 

25 

1 

75 

1 

95 

0 

80 ( 

1 

13 

1 

.25 

Storage.; • 

0. 

5 

0 

14 

. 

• • 


. • 

0 

09 





Warehouses, piers, etc.. . . 

0. 

75 

0 

21 

• 


• 

* * 

0 

13 






* In these columns allowance has been made for dust. 

t If glass enclosing globes are used, multiply by 1.5. 

t When special color matching units are used, double this figure. 

Note. The values given above are for usual conditions as regards color of 
walls and ceilings and size of room. If walls and ceiling are unusually dark, 
increase the values 10 to 15 per cent. 
















































370 


APPENDIX A 


TABLE 14. (Par. 121) 

Power Required for Flame-arc Lighting Systems 


Class of Service. 

Watts per 
Square Foot. 

Height above 
Floor, Feet. 

Forge and blacksmith shops. 

0.35 

30-40 

Foundries. 

0.45 

30-40 

Machine shops, large tools. 

0.40 

20-30 

Piers (closed). 

0.15 

25 

Power houses, engine room. 

0.35 

30-40 

Steel mills. 

0.20 to 0.35 

30 



Above figures allow for depreciation due to dust. 


TABLE 15. (Par. 123) 

* 

Sizes of Lighting Units for Various Mounting Heights 

(Direct Illumination) 


Height of Ceiling. 

Size of Unit, Watts. 

Tungsten 

Vacuum 

Type. 

Tungsten 

Gas-filled 

Type. 

Mercury- 
vapor Lamp.* 

Flame-arc 

Lamp. 

Up to 9 ft. 

40, 50, 60 

not used 

200 

not used 

9-11 ft. 

60 or 100 

75 or 100 

200 or 400 

not used 

11-16 ft. 

100 

100 or 200 

v 400 

not used 

16-20 ft. 

100 

200 or 300 

400 or 725 

all sizes 

20 ft. and above. 

100 

300 to 1000 

400 or 725 

all sizes 


* 200-watt lamp is 20-inch lamp, low-pressure type; 400-watt lamp is 50- 
inch lamp, low-pressure type; 725-watt lamp is quartz lamp. This is seldom 
used where it must be mounted lower than 18 feet. 































APPENDIX A 


371 


TABLE 16. (Par. 121) 

Power Required for Lighting with Cooper Hewitt Lamps * 


(Low-pressure type) f 


Class of Service. 

Watts per Sq.ft. 

Height above 
Floor. 

Clothing Mfg: 



Cutters. 

1.72 

10 

Hand sewing. 

0.74 

10 

Pressing. 

1.65 

10 

Cotton Mills: 


Preparing thread. 

0.65 

10-12 

Weaving coarse goods. 

0.45 

10-15 

Weaving fine goods. 

0.70 

10-14 

Finishing. 

0.64 

12 

Embroidery plants. 

1.50 

10-15 

Forge and blacksmithing shops. 

0.40-0.70 

10-20 

Foundries: 


Moulding. 

0.25-0.50 

15-25 

Casting. 

Cleaning. 

0.25-0.50 

15-25 

0.30 

10-20 

Glass Mfg.: 


Cutting. 

0.85 

12 

Inspection. 

1.30-2.00 

10-12 

Polishing and grinding. 

0.70 

20 

Machine shops: 



Small tools. 

0.90 

10-15 

Large tools. 

0.45-0.60 

20-50 

Punch presses. 

0.75 

12-35 

Grinding and polishing. 

0.75 

9-14 

Boiler shops. 

0.35-0.50 

20-35 

Assembling and erecting. 

0.30-0.80 

10-50 

Inspection. 

0.85 

10-12 

Printing plants: 



Composition (hand). 

1.75 

10-12 

Sterotyping. 

0.75 

10-12 

Press rooms. 

1.00 

10-15 

Power houses: 



Boiler rooms. 

0.50 

16-40 

Turbine rooms. 

0.35 

15-75 

Shipping and storage. 

0.25 

8-12 

Silk mills: 



Preparing thread. 

0.70-1.00 

8-12 

Weaving. 

1.00-1.25 

8-14 

Finishing. 

1.50 

10-12 

Varnishing. 

0.80 

8-10 

Finishing automobile bodies. 

2.00-2.50 

8-10 

Wood working. 

0.35-0.90 

8-15 


* From data published by W. A. D. Evans, Proc. Ilium. Eng. Soc., Sept., 1915. 
t Quartz lamps would give same illumination with about 75 per cent of 
power required for low-pressure lamps. 















































372 


APPENDIX A 


TABLE 17.* (Par. 125) 

Desirable Spacing for Direct Lighting Units 


Class of Service. 


1. Armories, auditoriums, churches, 

public halls, restaurants, ball rooms, 
theatres, etc. 

2. Factories: 

Ordinary work. 

3. Factories: 

Close work. 

4. Offices, libraries, school rooms: 

(Where desk lamps are used). 

(Where no desk lamps are used). . . 
(Where no desk lamps are used). . . 
(Where no desk lamps are used). . . 


5. Stores: 

6. Drafting rooms: 

(Where desk lamps are used). 

(Where no desk lamps are used). . . 
(Where no desk lamps are used). . . 
(Where no desk lamps are used). . . 


eiling Height, 
Feet. 

Spacing of 
Unit, Feet. 

12-16 

12-16 

over 16 

15-26 

8-11 

8-11 

11-15 

10-16 

over 15 

14-22 

8-11 

6-10 

11-15 

8-13 

over 15 

11-17 

10-20 

12-18 

9-12 

7-11 

12-16 

9-14 

over 16 

11-18 

8-11 

8-11 

11-15 

10-16 

over 15 

14-22 

10-20 

12-18 

9-12 

6- 8 

12-16 

8-10 

over 16 

10-15 


* Based on data published by Holophane Works of G. E. Co. 

Note. It is not desirable to use the widest spacing on the smallest ceiling 
height. 
















APPENDIX A 


373 


TABLE 18. (Par. 125) 

Desirable Spacing for Indirect and Semi-indirect Lighting 

Units * 


Class of Service. 

Ceiling 
Height, Feet. 

Spacing of 
Unit, Feet. 

Hanging 
Height, Feet.f 


• 

8 

12 

1.5 



10 

15 

2.0-2.5 

1. Offices, libraries, school 


12 

18 

2.5-3.0 

rooms, stores, banks ' 


14 

24 

3.5-4.0 



16 

28 

3 .5-4.5 



18 

36 

4.0-5.0 



20 

40 

5.0-6 


- 

8 

6 

1.5 



10 

7.5 

2.0-4 

2. Drafting rooms, opera- 


12 

9.0 

3 

ting rooms, sewing < 


14 

14.0 

3.5-4.0 

machine rooms 


16 

16 

4.0-5.0 



18 

18 

4.0-5.5 



20 

20 

5.5-6 


* From data published by National X-Ray Reflector Co. Values are max¬ 
imum and should not be exceeded. f Distance from ceiling to top of reflector. 

TABLE 19. (Par. 135) 

Illumination Intensities for Street Lighting * 


Class of Streets. 

Average Illu¬ 
mination Inten¬ 
sity, Ft.-candles 

Desirable Characteristic. 

Important avenues and 
heavy traffic streets 

0.5-1.0 

Ample light on building 
fronts 

Secondary business streets 

0.1-0.2 

i 

Ample light on building 
fronts 

City residence streets 

0.05-0.1 

Subdued light on building 
fronts 

Suburban highways. 

0.01-0.02 

Maximum light on roadway 

Suburban residence streets 

0.005-0.015 

Very subdued light on 
building fronts 


* From National Electric Light Assn., Report of Committee on Street 


Lighting, 1916. 

Note. Bright moonlight has an intensity of about 0.03 foot-candle. 






















374 


APPENDIX A 


TABLE 20. (Par. 165) 

Temperature Ratings and Overloads of Motors * * * § 


Temperature Rise, Degrees Centigrade. 


Motor. 

Continuous Duty. 

Short-time 

Duty.J 

Full 

Load. 

Overload. 

Mo¬ 
mentary 
Overload 
Per cent. 

Full 

Load. 

Mo¬ 
mentary 
Overload 
Per cent. 

A 

B 

Sustained. 

A 

B 

A 

B 

Direct current, open 










type: f 










Small (up to 1 h.p).. . 

40 

40 








Medium (J to 1 h.p.) 

40 

45 

25%-l hr. 

55 

60 

50 

55 § 

60 § 


Large (above 1 h.p.). 

40 

45 

25%-2 hr. 

55 

60 

50 

55 § 

60 § 


Direct current, protect- 










ed type: 










Medium. 

50 

55 




50 

55 § 

60 § 


Large. 

50 

55 




50 

55 § 

60 § 


Direct current, enclosed 








type: 










Small. 

55 

55 








Medium. 

55 

6011 




50 

55 § 

60 § 


Large. 

55 

6011 




50 

50 § 

60 § 


Induction, open type: 






Small. 

40 









Medium. 

40 

. • . 

25%-l hr. 

55 

• • . 

50 

55 


50 

Large. 

40 

. . . 

25%-2 hr. 

55 

. . . 

50 

55 

• • • 

50 

Indue., enclosed type: 










Small. 

55 









Medium and large.. . 

55 





50 

55 


50 


* Based on ratings adopted by the Electric Power Club, an association of 
the principal manufacturers of motors. 

Column A = temperature rise for all parts except commutator. 

Column B = temperature rise for commutator. 

t For this type of motor another rating is also used, giving 50 deg. Cent, 
rise with no overload guarantees. 

t The temperature rise for short-time duty is generally based upon carrying 
full load for two hours. For crane service the time is one-half hour. Other 
ratings are used for special purposes. 

§ For special non-combustible insulations A =75, B =80. 

II For constant speed; 55 for adjustable and varying speed motors. 













































APPENDIX A 


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HHH(N(N(NCO'tU5N(XiO 


lO *o 

OHINCOiCNOiOOiOOiOOOOiOOlOO 

HHiNiNMM^^CONOWiO 

t-H r-H rnH 


* Allows at least 25 per cent overload. 

































376 


APPENDIX A 


TABLE 22. (Par. 167) 

Current and Size of Wire for Three-phase Induction Motors 


Horse- 

Amperes, Full Load. 

Size of WiRE.f 

Rubber or Other Insulation. 

power. 










110 

220 

440 

550 

110 

220 

440 

550 


V. 

V. 

V. 

V. 

V. 

V. 

V. 

V. 

0.5* 

3.6 

1.8 

0.9 

0.7 

14 

14 

14 

14 

1.0* 

6.4 

3.2 

1.6 

1.3 

12 

14 

14 

14 

2.0* 

11.6 

5.8 

2.9 

2.3 

8 

14 

14 

14 

3.0* 

16.4 

8.2 

4.1 

3.3 

8 

10 

14 

14 

5.0 

26.8 

13.4 

6.7 

5.4 

6 

8 

12 

14 

7.5 

39.2 

19.6 

9.8 

7.9 

3 

8 

12 

12 

10.0 

53.2 

26.6 

13.3 

10.7 

1 

6 

8 

10 

15.0 

77.0 

38.6 

19.3 

15.5 

00 

3 

8 

8 

20.0 

109.0 

54.6 

27.3 

21.8 

000 

1 

6 

6 

25.0 

125.0 

62.6 

31.3 

25.1 

0,000 

o 

5 

6 

35.0 


85.6 

42.8 

34.3 


00 

2 

4 

50.0 


122.0 

61.0 

48.6 


0,000 

0 

1 

75.0 


179.0 

89.0 

72.0 


350,000 

00 

0 

100.0 


237.0 

118.0 

95.0 


500,000 

0 000 

OOO 

150.0 


353.0 

176.0 

141.0 


two 400,000 

400 000 

300 000 

200.0 


451.0 

226.0 

181.0 


two 500,000 

500 000 

400 000 

250.0 


560.0 

280.0 

224.0 


two 700 OOO 

700 000 

500 OOO 

300.0 


670.0 

335.0 

268.0 


two 900,000 

900,000 

600,000 


* These motors are thrown directly on the line; all others are provided with 
auto-starters set to give a starting torque equal to full-load running torque. 

t Sizes of wire are for squirrel-cage motors. For slip-ring motors, smaller 
sizes can be used. See paragraph 331. 






































APPENDIX A 


377 


TABLE 23. (Par. 167) 

Current and Size of Wire for Two-phase Induction Motors 


Horse- 

Amperes, Full Load, t 

Size of Wire t 

Rubber or Other Insulation. 

power. 





• 





110 • 

220 

440 

550 

110 

220 

440 

550 


V. 

V. 

V. 

V. 

V. 

V. 

V. 

V. 

0.5* 

3.2 

1.6 

0.8 

0.6 

14 

14 

14 

14 

1.0* 

5.6 

2.8 

1.4 

1.1 

14 

14 

14 

14 

2.0* 

10.0 

5.0 

2.5 

2.0 

10 

14 

14 

14 

3.0* 

14.4 

7.2 

3.6 

2.9 

8 

12 

14 

14 

5.0 

23.2 

11.6 

5.8 

4.7 

6 

8 

12 

14 

7.5 

34.0 

17.0 

8.5 

6.8 

4 

8 

12 

12 

10.0 

46.0 

23.0 

11.5 

9.2 

2 

6 

10 

10 

15.0 

66.8 

33.4 

16.7 

13.4 

0 

4 

8 

8 

20.0 

94.4 

47.2 

23.6 

18.9 

000 

2 

6 

S 

25.0 

108.4 

54.2 

27.1 

21.7 

000 

1 

5 

6 

35 0 


74.2 

37.1 

29.7 


0 

3 

5 

50 0 


105.0 

52.6 

42.1 


000 

1 

2 

75 0 


155.0 

77.3 

61.9 


300,000 

0 

1 

100 0 


205.0 

103.0 

82.0 


450,000 

000 

00 

150 0 


306.0 

153.0 

123.0 


two 300,000 

300,000 

0,000 

200 0 


390.0 

195.0 

156.0 


two 400,000 

400,000 

300,000 

250 0 


484.0 

242.0 

194.0 


two 500,000 

500,000 

400,000 

300.0 


580.0 

290.0 

232.0 


two 600,000 

600,000 

500,000 


* These motors are thrown directly on the line; all others are provided with 
auto-starters set to give a starting torque equal to full-load running torque. 

t Values of current are for a two-phase, four-wire system; if three wires 
are used, current in common wire would be 1.41 times value given. \ alues 
for single-phase motors would be double the values given in the table. 

% Sizes of wire are for squirrel-cage motors. For slip-ring motors, smaller 
sizes can be used. See paragraph 331. 









































378 


APPENDIX A 


TABLE 24. (Par. 168) 

Power Factor of Induction Motors * 


(Two and three-phase) 


Horsepower. 

Power 

Factor. 

f Load. 

Full Load. 

2.. 

0.71 

0.79 

5. 

0.82 

0.85 

10. 

0.79 

0.84 

20. 

0.83 

0.88 

50. 

0.85 

0.89 

100. 

0.87 

0.91 

200. 

0.94 

0.96 


* For 60 cycles; 25-cycle motors are practically the same. 


TABLE 25. (Par. 189) 
Usual Speed Ratings of Motors 



D.C. 

A.C., 60 Cycles. 

A.C., 

25 Cycles. 

Horse- 












power. 

s 

S 

§ 

2 


2 

s 


S 

£ 

s 


pi 

pi 

hi 

Pi 

Pi 

Pi 

pi 

Pi 

Pi 

Pi 

pi 


pi 

pi 

pi 

pi 

pi 

pi 

pi 

pi 

pi 

pi 

pi 

1 

1750 

1150 



1675 

1100 






1.5 

1750 

1150 



1690 

1120 






2 

1750 

1150 



1720 

1120 



1445 

710 


3 

1750 

1150 



1730 

1130 



1445 

720 


5 

1750 

1150 

800 


1735 

1140 

850 


1445 

720 


7.5 

1750 

1150 

850 


1740 

1140 

855 


1450 

725 


10 

1750 

1150 

800 


1740 

1150 

860 


1455 

725 


15 

1750 

1150 

850 

600 

1740 

1155 

860 

680 

1455 

730 

480 

20 

1750 

1150 

800 

600 

1740 

1160 

860 

685 

1465 

730 

485 

25 

1750 

1100 

825 

600 

1755 

1160 

865 

685 

1465 

730 

485 

30 

1750 

1150 

850 

675 

1755 

1160 

865 

685 

1465 

725 

480 

40 

1700 

1150 

800 

600 

1755 

1170 

870 

685 

1460 

725 

480 

50 

1700 

1150 

750 

570 

1755 

1170 

870 

690 

1460 

730 

485 


Note. Above are approximate full-load speeds. 

























































APPENDIX A 


379 


TABLE 26. (Par. 208) 

Standard Pulley Sizes for Motors * 


Motor Sizes. 


Pulley. 


H.P. 

R.P.M. 

H.P. 

R.P.M. 

H.P. 

R.P.M. 

H.P. 

R.P.M. 

Diam., 

Ins. 

Face, 

Ins. 





1 

1150 

2 

1750 

4 

3 





2 

1150 

3 

1750 

4 

3 





3 

1150 

5 

1750 

5 

4 





5 

1150 

7.5 

1750 

6 

4 



5 

800 

7.5 

1150 

10 

1750 

7 

5 



7.5 

850 

10 

1150 

15 

1750 

8 

6 



10 

800 

15 * 

1150 

20 

1750 

9 

7 



15 

850 

20 

1150 

30 

1750 

10 

8 

15 

600 

20 

800 

30 

1150 

40 

1700 

11 

10 

20 

600 

30 

850 

40 

1150 

50 

1700 

12 

11 

*to 

675 

40 

800 

50 

1150 



13 

12 

40 

600 

50 

750 

75 

1150 



14 

12 

50 

570 

75 

850 

100 

1150 



16 

13 


* Recommended by “ The Electric Power Club,” an association of the 
principal motor manufacturers. 

































380 


APPENDIX A 


TABLE 27. (Par. 208) 

Horsepower Transmitted per Inch of Width by Leather 
, Belts with Pulleys of Equal Diameters * 


Diam. of 
Pulley, Ins. 







Revolutions 

PER 

Minute. 









100 

200 

300 

400 

500 

600 

800 

1000 

1200 

1400 

1600 

1800 

2000 













Single Belts. 











3 

0 

11 

0 

19 

0 

28 

0 

38 

0 

47 

0 

.57 

0 

75 

0 

93 

1 

08 

1 

25 

1 

43 

1 

61 

1 

79 

4 

0 

13 

0 

.25 

0 

38 

0 

50 

0 

63 

0 

.75 

0 

99 

1 

22 

1 

45 

1 

68 

1 

91 

2 

13 

2 

33 

5 

0 

16 

0 

.31 

0 

47 

0 

63 

0 

79 

0 

.93 

1 

22 

1 

51 

1 

82 

2 

08 

2 

34 

2 

58 

2 

84 

6 

0 

19 

0 

.37 

0 

57 

0 

75 

0 

94 

1 

10 

1 

45 

1 

79 

2 

13 

2 

44 

2 

74 

3 

02 

3 

27 

8 

0 

25 

0 

.50 

0 

76 

0 

99 

1 

23 

1 

.46 

1 

90 

2 

34 

2 

73 

3 

11 

3 

44 

3 

72 

3 

93 

10 

0 

31 

0 

63 

0 

94 

1 

22 

1 

51 

1 

.80 

2 

33 

2 

84 

3 

27 

3 

66 

3 

94 

4 

43 

4 

22 

12 

0 

37 

0 

75 

1 

12 

1 

46 

1 

80 

2 

.13 

2 

74 

3 

28 

3 

70 

4 

02 

4 

20 

4 

21 



14 

0 

43 

0 

87 

1 

29 

1 

69 

2 

08 

2 

.45 

3 

11 

3 

65 

4 

03 

4 

21 

4 

17 





16 

0 

50 

0 

98 

1 

46 

1 

91 

2 

34 

2 

.74 

3 

44 

3 

93 

4 

20 

4 

17 







18 

0 

56 

1 

10 

1 

63 

2 

13 

2 

60 

3 

.02 

3 

72 

4 

13 

4 

21 









20 

0 

62 

1 

22 

1 

80 

2 

34 

2 

84 

3 

.27 

3 

94 

4 

22 











24 

0 

74 

1 

45 

2 

14 

2 

74 

3 

28 

3 

70 

4 

20 













28 

0 

86 

1 

68 

2 

45 

3 

11 

3 

66 

4 

02 

4 

17 













32 

0 

98 

1 

92 

2 

75 

3 

44 

3 

95 

4 

20 















36 

1 

10 

2 

14 

3 

03 

3 

72 

4 

43 

4 

21 















40 

1 

22 

2 

35 

3. 

28 

3 

93 

4. 

22 

















44 

1 

34 

2 

55 

3. 

52 

4. 

10 

4. 

19 

















48 

1 

45 

2 

74 

3. 

73 

4. 

20 



















52 

1 

57 

2 

93 

3. 

89 

4. 

23 



















56 

l 

68 

3 

11 

4. 

02 

4. 

17 



















60 

1. 

80 

3 

27 

4. 

13 






















* Adapted from National Electric Light Association, “ Salesman’s Handbook.” 






































APPENDIX A 


381 


TABLE 27 —Continued 


Revolutions per Minute. 


Diam. of 


Pulley, Ins. 

100 

200 

300 

400 

500 

600 

800 

1000 

1200 

1400 

1600 

1800 

2000 












Double Belts. 











3 

0 

.16 

0 

29 

0 

44 

0 

59 

0 

74 

0 

90 

1 

20 

1 

48 

1 

78 

2 

07 

2 

36 

2 

63 

2 

91 

4 

0 

.19 

0 

.39 

0 

59 

0 

79 

0 

98 

1 

20 

1 

58 

1 

97 

2 

35 

2 

73 

3 

09 

3 

45 

3 

78 

5 

0 

.24 

0 

49 

0 

74 

0 

.99 

1 

23 

1 

49 

1 

97 

2 

45 

2 

91 

3 

36 

3 

78 

4 

20 

4 

60 

6 

0 

.29 

0 

59 

0 

88 

1 

19 

1 

48 

1 

78 

2 

36 

2 

90 

3 

45 

3 

95 

4 

44 

4 

90 

5 

33 

8 

0 

.39 

0 

79 

1 

19 

1 

58 

1 

97 

2 

36 

3 

08 

3 

78 

4 

45 

5 

06 

5 

60 

6 

06 

6 

45 

10 

0 

49 

0 

99 

1 

49 

1 

98 

2- 

45 

2 

92 

3 

80 

4 

61 

5 

34 

5 

95 

6 

46 

6 

75 

6 

85 

12 

0 

59 

1 

19 

1 

78 

2 

37 

2 

90 

3 

45 

4 

45 

5 

33 

6 

08 

6 

60 

6 

83 

6 

81 



14 

0 

69 

1 

39 

2 

07 

2 

74 

3 

36 

3 

96 

5 

05 

5 

96 

6 

52 

6 

85 

6 

75 





16 

0 

79 

1 

58 

2 

35 

3 

10 

3 

79 

4 

45 

5 

59 

6 

45 

6 

83 

6 

75 







18 

0 

88 

1 

78 

2 

63 

3 

46 

4 

21 

4 

91 

6 

07 

6 

75 

6 

80 









20 

0 

98 

1 

97 

2 

91 

3 

80 

4 

61 

5 

34 

6 

47 

6 

86 











24 

1 

19 

2 

35 

3 

45 

4 

45 

5 

33 

6 

08 

6 

83 













28 

1 

39 

2 

72 

3 

95 

5 

06 

5 

96 

6 

63 

6 

74 













32 

1 

58 

3 

08 

4 

46 

5 

59 

6. 

46 

6 

84 















36 

1 

78 

3 

45 

4 

90 

6 

07 

6. 

75 

6 

81 















40 

1 

97 

3 

79 

5 

34 

6 

46 

6. 

86 

















44 

2 

16 

4 

13 

5. 

73 

6 

71 

6. 

78 

















48 

2 

35 

4 

45 

6. 

07 

6 

83 



















52 

2. 

54 

4 

76 

6. 

38 

6 

85 



















56 

2. 

73 

5. 

06 

6. 

60 

6 

74 



















60 

2. 

91 

5. 

29 

6. 

75 






















When pulleys are of unequal diameters and are approximately 15 ft. between 
centres, find the horse-power from the table, using the smaller size pulley 
diameter and speed and then multiply by the factor found below: 


Ratio of pulleys. . 

8 to 1 

7 to 1 

6 to 1 

5 to 1 

4 to 1 

3 to 1 

2 to 1 

Ea.ctor. 

0.85 

0.88 

0.90 

0.92 

0.94 

0.96 

0.98 



























































382 


APPENDIX A 


TABLE 28. (Par. 225) 

Dimensions of Iron Conduit and Elbows. 



Conduit 



/—fffi 


/ 

& 

f 

s 


1 


< —T—* 



Elbow 


Size. 

D 

Outside Diam. 

O 

Inside Diam. 

Weight 

per 

Foot, 

R 

Radius. 

T 

Offset. 

Weight 
per 100 





in 


Decimal. 

Nearest 

64th. 

Decimal. 

Nearest 

64th. 

Lbs. 


• 

Lbs. 

1 

0.540 

35 

64 

0.364 

a 

0.425 

4J 

71 

42 

3 

¥ 

0.675 

43 

64 

0.493 

ii 

0.568 

4! 

71 

53 

1 

I 

0.840 

54 

0.622 

a 

0.852 

4i 

7f 

75 

3 

4 

1.050 

1A 

0.824 

53 

64 

1.134 

5| 

8f 

120 

1 

1.315 

m 

1.049 

1* 

1.684 

5 i 

91 

200 

11 

1.660 

in 

1.380 

ia 

2.281 

71 

101 

300 

H 

1.900 

If! 

1.610 

1 39 

1 64 

2.731 

81 

12f 

427 

2 

2.375 

m 

2.067 


3.678 

91 

151 

700 

21 

2.875 

m 

2.469 

2a 

5.819 

101 

17f 

1300 

3 

3.500 

3fi 

3.068 

3& 

7.616 

13 

191 

1700 

31 

4.000 

4 

3.548 

035 

°64 

9.202 

15 

211 

2300 

4 

4.500 

m 

4.026 

4& 

10.889 

16 

221 

2700 

41 

5.000 

5 

4.506 

4 ff 

12.642 

18 

241 

3100 

5 

5.563 


5.047 

5A 

14.810 

24 

32 

5500 

6 

6.625 


6.065 


19.185 

30 

39! 

9000 


Conduit ia furnished in 10-foot lengths, threaded on both ends, with one 
coupling, 




































APPENDIX A 


383 


TABLE 29 (Par. 228) 

Dimensions of Locknuts and Bushings for Iron Conduit * 



t u » 

rf-H -*\ | 

i i 




All dimensions in inches. 


Size of 
Conduit. 

B 

D 

H 

0 

Style. 

B 

D 

T 

3 

8 

1 3 

16 

7 

8 

11 

32 

1 

2 

1 

29 

32 

1 J_ 

1 32 

5 

32 

1 

2 

15 

16 

1 

3 

8 

5 

8 

1 

1 

1-5- 

A 64 

5 

32 

3 

4 

1 7 

1 32 

1 -5- 

1 16 

7 

16 

13 

16 

1 

lj 

1 li 

A 32 

5 

32 

1 

1 15 

A 32 

A 16 

1 7 

32 


1 

1* 

111 

1 1 6 

5 

32 


113 

1 16 

1 15 

1 16 

19 

32 

1* 

2 

1 29 

A 3 2 

9_3_ 

"32 

5 

16 

U 

to 

“h 


5 

8 

1 £ 

1 8 

2 

21 

9_7_ 

Z 16 

11 

32 

2 

0-9- 

z 16 

21 

5 

8 

2 

2 

923 

"32 

915 

"16 

11 

32 

21 

CO 


25 

32 

21 

2 

^32 

Q15 

°32 

3 

8 

3 

3H 

Q15. 

'Jie 

27 

32 

3 

2 

0 29 
°32 

4^ 

13 

32 

31 

4* 

41 

^2 

7 

8 

31 

2 

4^ 

4M 

13 

32 

4 

41 

51 

15 

16 

4 

2 

51 

5» 

7 

16 

5 

61 

6 it 

U 

5 

2 

61 

6 H 

9 

16 


* “ Star ” bushings and locknuts, made by the Steel City Electric Co. 
















































384 


APPENDIX A 


TABLE 30. (Par. 229) 

Sizes of Iron Conduit for Rubber Covered Wires * 




Size of Conduit, Inches. 

Size of Wire, B. & S. 

Number of Wires 

in One Conduit. 

1 

2 

3 ‘ 

4 

14 f 

1 

2 

1 

2 

1 

2 

3 

4 

12 f 

1 

2 

3 

4 

y 

3 

4 

iot 

1 

2 

3 

4 

3 

4 

1 

8 

1 

2 

1 

1 

1 

6 

1 

2 

1 

11 

11 

5 

3 

4 

U 

11 

11 

4 

3 

4 

11 

11 

11 

3 

3 

4 

U 

11 

11 

2 

3 

4 

11 

11 

H 

1 

3 

4 

H 

11 

2 

0 

1 

U 

2 

2 

00 

1 ' 

2 

2 

Ol 

^2 

000 

1 

2 

2 

21 

0,000 

U 

2 

Ol 

^2 

21 

300,000 

11 

oi 

^2 

Ol 

"2 

3 

400,000 

11 

3 

3 

31 

500,000 

H 

3 

3 

31 

600,000 

H 

3 

31 


700,000 

2 

31 

31 


800,000 

2 

31 

4 


900,000 

2 

31 

4 


1,000,000 

2 

4 

4 


1,250,000 

oi 

2 

4 1 

41 

■ 

1,500,000 

OI 
" 2 

41 

5 


1,750,000 

3 

5 

5 


2,000,000 

3 

5 

6 


14 duplex f 

1 

2 

3 

4 

1 

1 

12 duplex f 

1 

2 

3 

4 

1 

11 

10 duplex f 

3 

4 

1 

H 

11 


* Adopted by the National Electric Contractors’ Association and specified 
by the National Electric Code. (Rule 28.) 

t These sizes are solid, all others are stranded. 

Note. Sizes are based on three elbows up to No. 10 and two for larger 
wires. For more elbows or long runs, the next larger size of conduit should be 
used when the maximum size of wire specified for a conduit is used. For 
sxample, three No. 1 wires should be installed in a 2-in. conduit for long runs. 















APPENDIX A 


385 


TABLE 31. (Par. 229) 


Sizes of Pull Boxes 


Size of Conduit, Ins. 

Length in 

Ins. for Number of Wires Given Below. 

1 Wire. 

2 Wires. 

3 Wires. 

4 Wires. 

Small 

Wires. 

1 . 

18 

12 

12 

12 - 

12 - 

H . 

24 

18 

18 

12 

12 

l* . 

30 

18 

18 

18 

12 

2 . 

36 

24 

24 

18 

12 

2 J. 

36 

24 

24 

24 

12 

3. 

48 

30 

30 

24 

18 

3£. 


36 

30 

30 

18 

4. 


36 

36 

AO 

18 

5. 


48 

36 


18 

6 . 



48 


18 







Where several wires are in the same conduit, the length can be made less, since 
each wire is of a smaller diameter and therefore can be more easily bent. 

See Fig. 133 for illustration of pull box. 


TABLE 32. (Par. 230) 

Spacing of Anchors on Vertical Runs * 


Size. 

Distance between 
Anchors. Feet. 

Nos. 14 to 0. 

100 

00 to 0000 . 

80 

0000 to 350,000 cir. mils. 

60 

350,000 to 500,000 cir. mils. 

50 

500,000 to 750,000 cir. mils. 

40 

over 750,000 cir. mils. 

35 


* National Electrical Code. 













































386 


APPENDIX A 


TABLE 33. (Par. 247) 


M-D-^ 



Dimensions of B. & D. Cleats * 


No. 

Sizes of Wires. 

Dimensions in Inches. 

A 

B 

c 

D 

E 



1 

14-6 

2 

11 

16 

t 

H 

1 

H 

10-2 

21 

B 

1- 1 

1A 

1 

2 

2-0 

21 

1A 

!- t 

1 ! 

1 

21 

0-000 

2f 

1A 

_ 1 

16 4 

IB 

1 

3 

000-200,000 c.m. 

3A 

•tre 

A~ B 

2 

1 

3j 

200,000-500,000 c.m. 

31 

1 -3- 

1 16 

B-H 

2 

1 

4 

500,000-1,000,000 c.m. 

31 

If 

l -H 

2f 

1 

4* 

1,000,000-2,000,000 c.m. 

51 

2 

1A-2 

3A 

1A 




Dimensions in Inches. 

1 








Size of 

No. 

Sizes of Wires. 




I 

Screw or 




G 

H 



Bolt. 



r 

Diam. 

Max. 

Min. 



1 

14-6 

5 

8 

U 

1 

4 

IB 

U 

No. 12 

H 

10-2 

1 

1A 

5 

16 

2A 

IB 

No. 14f 

2 

2-0 

3 

4 

U 

JL 

16 

21 

2 

No. 14f 

21 

0-000 

B 

1A 

5 

16 

2A 

21 

1" 

3 

000-200,000 c.m. 

1 

U 

3 

8 

21 

21 

A" 

31 

200,000-500,000 c.m. 

1 _JL 

1 16 

1A 

B 

3 

2f 

i" 

4 

500,000-1,000,000 c.m. 

1 & 

A 8 

1 A 

1 8 

B 

3f 

31 

1" 

41 

1,000,000-2,000,000 c.m. 

2 

2 

9 

16 

41 

4 

1" 


* Dimensions taken from samples, 
t Or 1-inch lag screw or machine bolt. 

Round head blued-wood screws or round-head machine screw.a with. nuta 


are generally used for sizes 1 to 2 inclusive. 
















































APPENDIX A 


387 


TABLE 34. (Par. 256) 

Dimensions of Bare Stranded Copper Cables * 

Concentric Stranding 


Size. 

Diam. 

of 

Cable. 

Inches 

Number 

of Wires in 

Cable. 

Resis., 
Ohms 
per 
1000 
Ft., 25° 
C. (77° 
Fahr.) 

127 

91 

61 

37 

19 

7 

Diameter of Wires, Mils. 

2,000,000 c.m. 

1.631 

125 5 

148.2 





0.00539 

1,900,000 “ 

1.590 

122 3 

144.5 

.... 

.... 



0.00568 

1,800,000 “ 

1.548 

119.1 

140.7 

.... 

.... 



0.00599 

1,700,000 “ 

1.504 

115 7 

136.7 

.... 




0.00634 

1,600,000 “ 

1.459 

112 2 

132.6 

162.0 

.... 



0.00674 

1,500,000 “ 

1.412 

108.7 

128 4 

156.8 

.... 



0.00719 

1,400,000 “ 

1.364 

150.1 

124 0 

151.5 

.... 



0.00770 

1,300,000 “ 

1.315 

101.2 

119.5 

146.0 

.... 



0.00830 

1,200,000 “ 

1.263 

97.2 

114 8 

140.3 

.... 



0.00899 

1,100,000 “ 

1.209 

93.1 

109 9 

134.3 

.... 



0.00981 

1,000,000 “ 

1.152 

• • • • 

104.8 

128 0 

164.4 



0.01080 

950,000 ‘ * 

1.123 


102.2 

124.8 

160.3 



0.0114 

900,000 4 4 

1.093 


99.5 

121 5 

156.0 



0.0120 

850,000 4 4 

1.062 


96.7 

118 0 

151.6 



0.0127 

800,000 4 4 

1.031 


93.8 

114 5 

147.1 



0.0135 

750,000 44 

0.998 


• • • • 

110 9 

142.4 



0.0144 

700,000 44 

0.964 


.... 

107 1 

137.5 



0.0154 

650,000 4 4 

0 929 


• • • • 

103 2 

132.6 



0.0166 

600,000 4 4 

0.893 



99 2 

127.4 



0.0180 

550,000 4 4 

0 855 



95.0 

121.9 

170.1 


0.0196 

500,000 44 

0 814 


. . . . 

90.5 

116.2 

162.2 


0.0216 

450,000 44 

0 772 


.... 

85.9 

110 3 

153.9 


0.0240 

400,000 44 

0.728 



81.0 

104 0 

145.1 


0.0270 

350,000 44 

0.681 


.... 

75.8 

97.3 

135.8 


0.0308 

300,000 44 

0 630 


.... 


90 0 

125.7 


0.0360 

250,000 4 4 

0.575 


.... 


82 2 

114.7 


0.0431 

0,000 B.&S 

0 528 


.... 

.... 

75.6 

105.5 


0.0509 

000 44 

0 470 


.... 

.... 

67.3 

94.0 


0.0642 

00 44 

0 418 


.... 

.... 

60.0 

83.7 

. . . 

0.0811 

0 44 

0 373 


• • • • 

.... 

53.4 

74.5 

128.8 

0.102 

1 44 

0 332 


... * 

.... 

.... 

66.4 

109.3 

0.129 

2 44 

0.292 


.... 

.... 

• • • • 

59.1 

97 4 

0.162 

3 44 

0 260 



• • • • 

• . . . 

52.6 

86 7 

0.205 ' 

4 44 

0 232 


.... 

• • • • 

• • • • 

46.9 

77.2 

0.259 

5 44 

0 206 


.... 

• • • • 

• • . . 

41.7 

68 8 

0.326 

6 44 

0 184 


.... 

• • • • 

• • . • 

• • • • 

61.2 

0.410 

7 44 

0 164 


• • • • 

• • • • 

• • . • 

• . . • 

54.5 

0.519 

8 44 

0.146 

.... 

• • • • 

• * * * 

• * * * 

• * * * 

48 6 

0.654 


* From the Bureau of Standards, Circular No. 31. 

Note. Figures in black-face type apply to standard stranding. 






















































388 


APPENDIX A 


TABLE 35. (Par. 257.) 

Dimensions of Insulated Wires * 

With N. E. Code thickness of insulation for 0-600 volts. 


Size. 


Rubber Insulation. 


Thickness 
of Insula- 



tion, Ins. 

Mils. 

Ins. 

Mils. 

f 18 B. & S. 

At 

112 

7 

64 

154 

fie 

i i 

At 

155 

5 

32 

197 

fl4 

i i 

3 

64 

208 

13 

64 

258 

tl2 

11 

3 

64 

225 

7 

32 

275 

no 

i l 

3 

64 

246 

1 

4 

296 

8 

< c 

3 

64 

290 

19 

64 

340 

6 

i l 

1 

16 

360 

23 

64 

410 

5 

( i 

1 

16 

396 

25 

64 

460 

4 

i i 

1 

16 

422 

27 

64 

486 

3 

i < 

1 

16 

451 

29 

64 

515 

2 

( C 

1 

16 

504 

1 

2 

588 

1 

i l 

5 

64 

571 

37 

64 

655 

0 

i c 

5 

64 

613 

39 

64 

697 

00 

L i 

5 

64 

659 

21 

32 

743 

000 

i i 

5 

64 

709 

45 

64 

793 

0,000 

i ( 

5 

64 

767 

49 

64 

851 

250,000 

c.m. 

3 

32 

845 

27 

32 

929 

300,000 

i L 

3 

32 

902 

29 

32 

986 

350,000 

( i 

3 

32 

952 

61 

64 

1036 

400,000 

C i 

3 

32 

1001 

1 

1085 

450,000 

l ( 

3 

32 

1044 

1-3_ 

1 64 

1128 

500,000 

i c 

3 

32 

1087 

1 -2- 
A 32 

1171 

550,000 

i ( 

7 

64 

1157 

1 32 

1241 

600,000 

(( 

7 

64 

1194 

1 _3_ 

1 1 6 

1278 

650,000 

(c 

7 

64 

1231 

1 1A 

1 64 

1315 

700,000 

(< 

7 

64 

1266 

111. 

1 64 

1350 

750,000 

( c 

7 

64 

1300 

119 

1 64 

1384 

800,000 

(i 

7 

64 

1333 

111 

1 64 

1417 

850,000 

t i 

A 

1365 

1 23 

1 64 

1449 

900,000 

«( 

7 

64 

1395 

Iff 

1479 

950,000 

( i 

7 

64 

1425 

Iff 

1509 

1 ,000,000 

< l 

7 

64 

1455 

1 29 

1 64 

1539 

1,250,000 

( i 

1 

8 

1623 

1 £ 

1 8 

1707 

1,500,000 

(< 

1 

8 

1747 

1 1 

4 

1831 

1,750,000 

(< 

1 

8 

1860 

1 55 

1 6 4 

1944 

2 ,000,000 

“ 1 

1 

8 

1965 

1 31 

1 32 

2049 


Outside Diameter. 


Single Braid. 


Double Braid. 


Ins. 


5 

32 
1 3 
64 

I 
4 

9 

32 

19 

64 

II 
32 
1 3 
32 
1 5 

32 

31 
64 

33 
64 
19 

32 
21 
32 
45 
64 

3 

4 

5_1 

64 

55 
64 
1 5 
16 

63 

64 
1 _A_ 
1 32 

n 

in 

u 
1 - 2 - 
-*-3 2 
1 _ 5 _ 

A 16 
1 23. 
A 64 
125 
-*-6 4 
1 27 
A 64 
-I 29 
1 64 

Iff 

Iff 

1 JJ- 

1 32 

Iff 

Iff 
161 
-*-6 4 
0_3_ 
"64 


* Standard Underground Cable Co. 
t These sizes are solid. All others are stranded, 
t Fixture wire. 


Slow-burn¬ 
ing Insula¬ 
tion. 

Diam., Ins. 


3 

16 

7 

32 

1 

4 

9 

32 
1 1 

32 
3 
8 

27 

64 

29 

64 

31 

64 

33 
64 
37 
64 
41 
64 
49 
64 
53 
64 

7 

8 

1 5 
16 

31 

32 
1 JL 
1 3 2 

1* 

Iff 


1-9- 

•*-32 


-^ 27 


32 


1 33 
-*-6 4 

1 - 3 - 

-*-16 

1 39 
a 64 
1 11 
A T6 

A 4 
A 8 






















APPENDIX A 


389 


TABLE 36. (Par. 261) 
Current-carrying Capacity of Wires 


0-600 volt insulation, N. E. C. Standard for interior wiring 


Size, 

Circ. Mils. 

Size, 

B. & S. Gauge. 

Rubber Insulation, 
Amperes. 

A. 

Other Insulations, 
Amperes. 

B. 

1,620 

18* 

3 

5 

2,580 

16* 

6 

10 

4,110 

14 

15^ 

20 

6,530 

12 

20 

25 

10,400 

10 

25 

30 

16,500 

8 

35 

50 

26,300 

6 '' 

50*-" 

70 

33,100 

5 

55 

80 

41,700 

4 

70 

90 

52,600 

3 

80- 

100 

66,400 

2 

90 

125 

83,700 

1 

100 

150 

106,000 

0 

125 

200 

133,000 

00 

150 

225 

168,000 

000 

175 

275 

212,000 

0000 

225/ 

325 

300,000 


275 

400 

400,000 


325 

500 

500,000 


400 

600 - 

600,000 


450 

680 

700,000 


500 

760 

800,000 


550 

840 

900,000 


600 

920 

1 ,000,000 


650 

1000 

1 ,100,000 


690 

1080 

1 ,200,000 


730 

1150 

1,300,000 


770 

1220 

1,400,000 


, 810 

1290 

1,500,000 


850 

1360 

1,600,000 ' 


890 

1430 

1,700,000 


930 

1490 

1,800,000 


970 

1550 

1,900,000 


1010 

1610 

2 ,000,000 


1050 

1670 


Note.—V oltage drop is not considered in the above table. 

* Wires smaller than No. 14 B. & S. gauge should not be used except for fixture 


wiring and pendant cords. 



































390 


APPENDIX A 


TABLE 37. (Par. 295) 


Dimensions of Lighting Panel Board Cabinets * 


125 VOLT.f 


250 VoLT.f 



Circuits. 

Style 

A. 

Style 

B. 

Style C. 

Style D. 

Style 

A. 

Style C. 

Height. 

1 Width. 

Height. 

Width. 

Height. 

Width. 

Height. 

Width. 

Height. 

Width. 

Height. 

* 

Width. 


2 

15.5 

23 

15.5 

23 

15.5 

19 


15.5 

19 


17.5 

25 

17.5 

19 


4 

19.5 

23 

19.5 

23 

19.5 

19 


19.5 

19 


19.5 

25 

19.5 

19 


6 

21.5 

23 

21.5 

23 

21.5 

19 


21.5 

19 


23.5 

25 

23.5 

19 


8 

23.5 

23 

25.5 

23 

23.5 

19 


25.5 

19 


27.5 

25 

27.5 

19 


10 

25.5 

23 

27.5 

23 

25.5 

19 


27.5 

19 


29.5 

25 

29.5 

19 

rd 

o 

12 

29.5 

23 

31.5 

23 

29.5 

19 


31.5 

19 


33.5 

25 

33.5 

19 

0 

a 

14 

31.5 

23 

35.5 

23 

31.5 

19 


35.5 

19 


35.5 

25 

35.5 

19 

m 

16 

35.5 

23 

37.5 

23 

35.5 

19 


37.5 

19 


39.5 

25 

39.5 

19 

o 

18 

37.5 

23 

41.5 

23 

37.5 

19 


41.5 

19 


43.5 

25 

43.5 

19 

3 

20 

39.5 

23 

43.5 

23 

39.5 

19 


43.5 

19 


45.5 

25 

45.5 

19 

o 

Q 

22 

41.5 

23 

47.5 

23 

41.5 

19 


47.5 

19 


49.5 

25 

49.5 

19 


24 

45.5 

23 

49.5 

23 

45.5 

19 


49.5 

19 


53.5 

25 

53.5 

19 


26 

47.5 

23 

53.5 

23 

47.5 

19 


53.5 

19 


57.5 

25 

57.5 

19 


28 

49.5 

23 

55.5 

23 

49.5 

19 


55.5 

19 


59.5 

25 

59.5 

19 


30 

51.5 

23 

59.5 

23 

51.5 

19 


59.5 

19 


63.5 

25 

63.5 

19 


32 

55.5 

23 

61.5 

23 

55.5 

19 


61.5 

19 


67.5 

25 

67.5 

19 


2 

19.5 

18 

19.5 

18 

19.5 

16. 

5 

19.5 

16. 

5 

19.5 

19 

19.5 

16.5 


3 

21.5 

18 

21.5 

18 

21.5 

16. 

5 

21.5 

16. 

5 

23.5 

19 

23.5 

16.5 


4 

23.5 

18 

25.5 

18 

23.5 

16. 

5 

25.5 

16. 

5 

27.5 

19 

27.5 

16.5 

M 

5 

25.5 

18 

27.5 

18 

25.5 

16. 

5 

27.5 

16. 

5 

29.5 

19 

29.5 

16.5 

o 

0 

6 

29.5 

18 

31.5 

18 

29.5 

16. 

5 

31.5 

16. 

5 

33.5 

19 

33.5 

16.5 

0 

7 

31.5 

18 

33.5 

18 

31.5 

16. 

5 

33.5 

16. 

5 

35.5 

19 

35.5 

16.5 

PQ 

8 

33.5 

18 

37.5 

18 

33.5 

16. 

5 

37.5 

16. 

5 

39.5 

19 

39.5 

16.5 

( V 

% 

9 

35.5 

18 

39.5 

18 

35.5 

16. 

5 

39.5 

16. 

5 

43.5 

19 

43.5 

16.5 

a 
• «-* 

10 

39.5 

18 

43.5 

18 

39.5 

16. 

5 

43.5 

16. 

5 

45.5 

19 

45.5 

16.5 


11 

41.5 

19 

47.5 

19 

41.5 

16. 

5 

45.5 

16. 

5 

49.5 

19 

49.5 

16.5 


12 

45.5 

19 

49.5 

19 

45.5 

16. 

5 

49.5 

16. 

5 

53.5 

19 

53.5 

16.5 


13 

47.5 

19 

53.5 

19 

47.5 

16. 

5 

53.5 

16. 

5 

55.5 

19 

55.5 

16.5 


“ Double branch ” have circuits bcfth sides of bus bars; “ single branch ” 
on one side. 

Style A—Enclosed fuses and knife switches on branches, no main fuses or 
switch. 

Style B—Plug fuses and knife switches on branches, no main fuses or switch. 

Style C—Enclosed fuses, no switches on branches, no main fuses or switch! 

Style D—Plug fuses, no switches on branches, no main fuses or switch. 

All boxes are 4| inches deep outside, not including trim. 

Dimensions given above are for steel boxes with a gutter 3 inches wide on 
top, bottom and sides and with a slate lining. 

* Crouse-Hinds Co. 

t Above sizes apply to panels with two-wire mains. Panels with three-wire 
mains are same height and about 2 inches wider. 



































































APPENDIX A 


391 


TABLE 38. (Par. 313) 
Demand Factors for Motor Loads * 


No. of Motors. 

Character of Load. 

Demand Factor. 

1 

Individual drives—tools, etc. 

1.25 

2 

Individual drives—tools, etc. 

1.00 

3 

Individual drives—tools, etc. 

0.75 to 0.85 

5 

Individual drives—tools, etc. 

0.60 to 0.70 

10 

Individual drives—tools, etc. 

0.40 to 0.50 

20 

Individual drives—tools, etc. 

0.40 

1 

Group drives 

1.25 

2 or more 

Group drives 

0.70 to 0.75 

1 

Fans, compressors, pumps, etc. 

1.25 

2 or more 

Fans, compressors, pumps, etc. 

0.85 to 1.00 


The above values make no allowance for future increase in the load. 
* Ratio of maximum load to connected load. 


TABLE 39. (Par. 315) 


Sizes of Fuses for Motors * 


Horse¬ 

power. 

D.C. 

115 

V. 

230 

V. 

1 

2 

8 

4 

1 

12 

6 

2 

20 

12 

3 

35 

20 

5 

55 

30 

71 

80 

40 

10 

100 

50 

15 

150 

80 

20 

200 

100 

25 

250 

125 

30 

300 

150 

35 

325 

175 

40 

400 

200 

50 

450 

225 


* These values are 



Two-phase A.C.f 

Three-phase A.C.f 

550 

110 

220 

440 

550 

110 

220 

440 

550 

V. 

V. 

V. 

V. 

V. 

V. 

V. 

V. 

V. 

3 

5 

3 

3 

3 

6 

3 

3 

3 

3 

9 

5 

3 

v 3 

10 

5 

3 

3 

5 

15 

8 

4 

3 

20 

9 

5 

4 

7 

25 

12 

6 

5 

25 

15 

7 

5 

12 

35 

20 

9 

7 

45 

25 

12 

9 

20 

60 

30 

15 

12 

60 

35 

15 

12 

25 

70 

35 

20 

15 

85 

45 

20 

20 

35 

100 

50 

25 

20 

125 

65 

30 

25 

45 

150 

70 

40 

30 

175 

80 

40 

35 

50 

175 

85 

45 

35 

200 

100 

50 

40 

65 

70 

80 

100 


120 

60 

50 


125 

65 

55 


175 

80 

65 


200 

100 

75 


for motors for constant speed or adjustable speed service. 


For varying speed service (cranes, etc.), see par. 315. 

t These values are for slip-ring motors and for the “ running fuses for 
squirrel-cage motors. The “starting fuses” should be based on carrying 
capacity of wire. (See par. 331.) Where small motors are thrown directly 
upon the line, “ starting fuses ” only are used unlqss a two-throw switch is 
provided which cuts out the “ running fuses ” when starting. 













































392 


APPENDIX A 


TABLE 40. (Par. 317) 


Values of Maximum Voltage Drop Allowable 



Per Cent. 

Voltage Drop between Wires for 


120 Volts. 

240 Volts. 

Lighting circuits: 
Branches. 

1.5 

1.8 

3.6 

Mains. 

1.0 

1.2 

2.4 

Feeders. 

2.5 

3.0- 

6 .0- 

Total. 

5.0 

6.0 

12.0 



Per 

Voltage Drop between Wires for 


Cent. 

110 V. 

220 V. 

440 V. 

550 V. 

« 

Power circuits: 

Branches. 

2.0 

2.2 

4.4 

8.8 

11.0 

Mains. 

2.0 

2.2 

4.4 

8.8 

11.0 

Feeders. 

6.0 

6.6 

13.2 

26.4 

33.0 

Total. 

10.0 

11.0 

22.0 

44.0 

55.0 


Above values are for the drop to the farthest lamp or motor. 


TABLE 41. (Par. 319) 

Branch Lighting Circuits 

Maximum length of circuit for 1.5 per cent drop. 


Distances in Feet.* 


Size, 

B. & S. 

120 Volts. 

240 Volts. 

Gauge. 

12 

10 

6 

5 

6 

5 

3 

2.5 


Amp. 

Amp. 

Amp. 

Amp. 

Amp. 

Amp. 

Amp. 

Amp. 

14 

29 

35 

58 

70 

116 

138 

232 

276 

12 

46 

55 

92 

110 

184 

220 

366 

440 

10 

73 

87 

146 

174 

292 

348 



8 

116 

139 

232 

278 

464 





* Note that the distance given is in each case to the load centre and not 
to the end of the run. 

































































APPENDIX A 


393 


TABLE 42. (Par. 323) 


Skin Effect for Round Copper Conductors 

Product of Cir. Mils and 

Factor. 

Cycles per Second. 

10 ,000,000 

1.00 

20 ,000,000 

1.01 

30,000,000 

1.02 

40,000,000 

1.04 

50,000,000 

1.06 

60,000,000 

1.09 

70,000,000 

1.12 

80,000,000 

1.15 

90,000,000 

1.18 

100 ,000,000 

1.22 

125,000,000 

1.31 

150,000,000 

1.41 

175,000,000 

1.50 

200 ,000,000 

1.59 

To find the skin effect, multiply the frequency by the size in circular mils. 
Find the corresponding factor from the table and multiply the direct-current 
resistance (given in Table 34) by this factor. The product will be the alter- 

nating current resistance. Thus for a 1,000,000 

c.m. cable at 60 cycles, the 

factor is 1.09. Hence the resistance per 1000 ft., 

is 1.09 X0.0108 =0.0118 ohm. 


The table is correct for either stranded or solid wires. 


394 


APPENDIX A 


TABLE 43. (Par 324) 
Power Factors of Apparatus 


* 

Power Factor. 

Incandescent lamps and heaters. 

1.0 

Arc lamps (multiple). 

0.60 

Cooper Hewitt lamps. 

0.85 

Induction motors—up to 15 hp. (running). 

0.80 

Induction motors—above 15 hp. (running). 

0.85 

Induction motors—starting. 

0.50 



Note. The values given for induction motors take into account the fact 
that as a rule the motors would not be carrying full load. The values refer 
to the sizes of individual motors. That is, if a feeder carries one 15-hp. and 
four 10-hp. motors, the total connected load is 55 hp., but the power factor 
should be taken as 0.80. 


TABLE 44. (Par. 330) 


Reactive and Resistance Factors 


Power 

Factor. 

Reactive 

Factor. 

Resistance 

Factor. 

Power 

Factor. 

Reactive 

Factor. 

Resistance 

Factor. 

1.00 

0 

1 

00 

0 

84 

0 

54 

0 

84 

0.99 

0.14 

0 

99 

0 

82 

0 

57 

0 

82 

0.98 

0.20 

0 

98 

0 

80 

0 

60 

0 

80 

0.97 

0.24 

0 

97 

0 

75 

0 

66 

0 

75 

0.96 

0.28 

0 

96 

0 

70 

0 

71 

0 

70 

0.95 

0.31 

0 

95 

0 

65 

0 

76 

0 

65 

0.94 

0.34 

0 

94 

0 

60 

0 

80 

0 

60 

0.93 

0.37 

0 

93 

0 

55 

0 

84 

0 

55 

0.92 

0.39 

0 

92 

0 

50 

0 

GO 

0 

50 

0.91 

0.41 

0 

91 

0 

45 

0 

89 

0 

45 

0.90 

0.44 

0 

90 

0 

40 

0 

92 

0 

40 

0.88 

0.48 

0 

88 

0 

35 

0 

94 

0 

35 

0.86 

0.51 

0 

86 

0 

30 

0 

95 

0 

o 

CO 

0.85 

0.53 

0 

00 

Or 

































APPENDIX A 


395 


TABLE 45. (Par. 334) 

Ratio of Reactance to Resistance 



Ratios for Distance between Wires of 

- 

Size of Wire 

In Con¬ 
duit. 

2\ Ins. 

4 Ins. 

5 Ins. 

6 Ins. 

8 Ins. 

12 Ins. 


60 Cycles 


10 

0 

.05 

0 

.09 

0.10 

0 

.11 

0 

.11 

0 

.12 

0 

.13 

8 

0 

.08 

0 

.13 

0.15 

0 

16 

0 

17 

0 

.18 

0 

.19 

6 

0 

.12 

0 

.21 

0.23 

0 

.24 

0 

26 

0 

.27 

0 

29 

5 

0 

.14 

0 

.25 

0.28 

0 

.30 

0 

31 

0 

33 

0 

36 

4 

0 

.15 

0 

.30 

0.34 

0 

36 

0 

38 

0 

41 

0 

44 

3 

0 

22 

0 

.37 

0.42 

0 

.45 

0 

47 

0 

50 

0 

54 

2 

0 

.26 

0 

.45 

0.52 

0 

.55 

0 

57 

0 

62 

0 

67 

1 

0 

32 

0 

.54 

0.62 

0 

67 

0 

70 

0 

75 

0 

82 

0 

0 

38 

0 

.66 

0.77 

0 

.82 

0 

86 

0 

92 

1 

01 

00 

0 

54 

0 

.80 

0.93 

0 

99 

1 

04 

1 

13 

1 

25 

000 

0 

64 

0 

97 

1.14 

1 

21 

1 

28 

1 

38 

1 

53 

0000 

0 

76 

1 

17 

1.38 

1 

48 

1 

56 

1 

70 

1 

87 

300,000 

1. 

01 

1 

54 

1.84 

1 

98 

2 . 

10 

2 

28 

2 

52 

400,000 

1. 

49 

1 

93 

2.33 

2 . 

52 

2 . 

67 

2 . 

92 

3 

26 

500,000 

1. 

75 

2 

30 

2.80 

3. 

03 

3. 

22 

3. 

54 



600,000 

1. 

85 

2 . 

52 

3.10 

3. 

40 

3. 

63 





700,000 

2 . 

06 

2 . 

84 

3.54 









800,000 

2 . 

49 

3. 

12 










900,000 

2 . 

69 

3. 

39 










1 ,000,000 

2 . 

89 

3. 

66 



































396 


APPENDIX A 


TABLE 45 —Continued 



i 

Ratios for Distance between Wires of 

Size of Wires. 

In Con¬ 
duit. 

2\ Ins. 

4 Ins. 

5 Ins. 

6 Ins. 

8 Ins. 

12 Ins. 


40 Cycles 


10 

0.03 

0 

06 

0 

07 

0 

07 

0 

07 

0 

08 

0 

09 

8 

0.05 

0 

09 

0 

10 

0 

11 

0 

11 

0 

12 

0 

13 

6 

0.08 

0 

14 

0 

15 

0 

16 

0 

17 

0 

18 

0 

19 

5 

0.09 

0 

17 

0 

19 

0 

20 

0 

21 

0 

22 

0 

24 

4 

0.10 

0 

20 

0 

23 

0 

24 

0 

25 

0 

27 

0 

29 

3 

0.15 

0 

25 

0 

28 

0 

30 

0 

31 

0 

33 

0 

36 

2 

0.17 

0 

30 

0 

35 

0 

37 

0 

38 

0 

41 

0 

45 

1 

0.21 

0 

36 

0 

41 

0 

45 

0 

47 

0 

50 

0 

55 

0 

0.25 

0 

44 

0 

51 

0 

55 

0 

57 

0 

61 

0 

67 

00 

0.36 

0 

53 

0 

62 

0 

66 

0 

69 

0 

75 

0 

83 

000 

0.43 

0 

65 

0 

76 

0 

81 

0 

85 

0 

92 

1 

02 

0,000 

0.51 

0 

78 

0 

92 

0 

99 

1 

04 

1 

13 

1 

25 

300,000 

0.67 

1 

02 

1 

22 

1 

32 

1 

40 

1 

52 

1 

67 

400,000 

1.00 

1 

28 

1 

55 

1 

68 

1 

77 

1 

95 

2 

17 

500,000 

1.17 

1 

53 

1 

87 

2 

02 

2 

14 

2 

36 

2 

63 

600,000 

1.23 

1 

68 

2 

07 

2 

27 

2 

42 

2 

67 

3 

02 

700,000 

1.38 

1 

90 

2 

36 

2 

58 

2 

76 

3 

05 

3 

45 

800,000 

1.67 

2 

08 

2 

63 

2 

87 

3 

08 

3 

40 



900,000 

1.80 

2 

26 

2 

87 

3 

15 

3 

38 





1 ,000,000 

1.93 

2 

45 

3 

08 

3 

40 

3 

67 
























APPENDIX A 


397 


TABLE 45 —Continued 



Ratios for 

Distance between Wires of 


Size of Wire. 

In Con¬ 
duit. 

2\ Ins. 

4 Ins. 

5 Ins. 

6 Ins. 

8 Ins. 

12 Ins. 


25 Cycles 


10 

0 

02 

0 

04 

0 

04 

0 

05 

0 . 

05 

0 

05 

0 

05 

8 

0 

03 

0 

05 

0 

06 

^0 

07 

0 . 

07 

0 

08 

0 . 

08 

6 

0 

05 

0 

09 

0 

10 

0 

10 

0 . 

11 

0 

11 

0 

12 

5 

0 

06 

0 

10 

0 

12 

0 

13 

0 . 

13 

0 

14 

0 

15 

4 

0 

06 

0 

12 

0 

14 

0 

15 

0 

16 

0 

17 

0 

18 

3 

0 

09 

0 

15 

0 

18 

0 

19 

0 

20 

0 

21 

0 

23 

2 

0 

11 

0 

19 

0 

22 

0 

23 

0 

24 

0 

26 

0 

28 

1 

0 

13 

0 

23 

0 

26 

0 

28 

0 

29 

0 

31 

0 

34 

0 

0 

16 

0 

28 

0 

32 

0 

34 

0 

36 

0 

38 

0 

42 

00 

0 

23 

0 

33 

0 

39 

0 

41 

0 

43 

0 

47 

0 

52 

000 

0 

27 

0 

.40 

0 

48 

0 

51 

0 

53 

0 

58 

0 

64 

0,000 

0 

32 

0 

49 

0 

58 

0 

62 

0 

65 

0 

71 

0 

78 

300,000 

0 

42 

0 

64 

0 

77 

0 

.83 

0 

88 

0 

95 

1 

05 

400,000 

0 

62 

0 

81 

0 

97 

1 

05 

1 

11 

1 

24 

1 

36 

500,000 

0 

73 

0 

96 

1 

17 

1 

26 

1 

34 

1 

48 

1 

65 

600,000 

0 

77 

1 

05 

1 

29 

1 

42 

1 

51 

1 

67 

1 

.88 

700,000 

0 

86 

1 

18 

1 

47 

1 

61 

1 

72 

1 

.91 

2 

.15 

800,000 

1 

03 

1 

30 

1 

64 

1 

79 

1 

92 

2 

12 

2 

.31 

900,000 

1 

12 

1 

42 

1 

79 

1 

96 

2 

11 

2 

.34 

2 

.49 

1 ,000,000 

1 

20 

1 

53 

1 

92 

2 

12 

2 

29 

2 

54 

























398 


APPENDIX A 


TABLE 46. (Par. 334) 
Drop Factors * 


Ratio of Reactance 


Drop Factors for Power Factors of 


to Resistance. 



1 

.00 

0 

.95 

0 

.90 

0 

.85 

0 

.80 

0 

.70 

0 

.60 

0 

.40 

0.1 

1 

.00 

1 

.00 

1 

.00 

0 

.94 

0 

.88 

0 

.80 

0 

.70 

0 

.60 

0.2 

1 

.00 

1 

.01 

1 

.01 

0 

.98 

0 

.92 

0 

.86 

0 

.82 

0 

.67 

0.3 

1 

.00 

1 

.05 

1 

.05 

1 

.02 

0 

99 

0 

93 

0 

.89 

0 

.74 

0.4 

1 

.00 

1 

.08 

1 

.10 

1 

.08 

1 

.04 

1 

00 

0 

.93 

0 

82 

0.5 

1 

.00 

1 

.11 

1 

.14 

1 

.13 

1 

10 

1 

07 

1 

01 

0 

92 

0.6 

1 

.01 

1 

15 

1 

.18 

1 

19 

1 

15 

1 

14 

1 

.09 

1 

01 

0.7 

1 

.02 

1 

18 

1 

.23 

1 

24 

1 

21 

1 

20 

1 

17 

1 

11 

0.8 

1 

02 

1 

21 

1 

.28 

1 

29 

1 

28 

1 

27 

1 

24 

1 

20 

0.9 

1 

03 

1 

25 

1 

.33 

1 

34 

1 

34 

1 

35 

1 

32 

1 

29 

1.0 

1 

04 

1 

28 

1 

.37 

1 

39 

1 

40 

1 

41 

1 

39 

1 

38 

1.1 

1 

05 

1 

32 

1 

41 

1 

44 

1 

45 

1 

48 

1 

47 

1 

46 

1.2 

1 

06 

1 

35 

1 

46 

1 

50 

1 

51 

1 

55 

1 

54 

1 

55 

1.3 

1 

07 

1 

39 

1 

51 

1 

55 

1 

57 

1 

62 

1 

63 

1 

64 

1.4 

1 

08 

1 

43 

1 

55 

1 

61 

1 

64 

1 

70 

1 

71 

1 

72 

1.5 

1 

10 

1 

47 

1 

60 

1 

67 

1 

70 

1 

77 

1 

80 

1 

81 

1.6 

1 

10 

1 

51 

1 

65 

1 

74 

1 

77 

1 

85 

1 

87 

1 . 

90 

1.7 

1 

13 

1 

55 

1 

70 

1 

79 

1 

84 

1 

92 

1 

95 

1 . 

99 

1.8 

1 

15 

1 

59 

1 

76 

1 

85 

1 

91 

1 

99 

2 

04 

2. 

08 

1.9 

1 

17 

1 

63 

1 

82 

1 

91 

1 

98 

2 

06 

2 

11 

2 

16 

2.0 

1 

18 

1 

68 

1 

87 

1 

96 

2 

04 

2 

14 

2 

19 

2 

25 

2.1 

1 

20 

1 

to 

1 

.92 

2 

03 

2 

10 

2 

21 

2 

28 

2 

35 

2.2 

1 

22 

1 

77 

1 

98 

2 

09 

2 

17 

2 

29 

2 

37 

2 

45 

2.3 

1 

23 

1 

82 

2 

03 

2 

15 

2 

23 

2 

37 

2 

45 

2 

53 

2.4 

1 

25 

1 

oo 

2 

o 

_ «© 

2 

22 

2 

30 

2 

44 

2 

53 

2 

62 


* Electric Journal, Vol. IV, p. 229. 
























APPENDIX A 


399 


TABLE 46— Continued. 


Ratio of Reactance 


Drop Factors for Power Factors of 


to Resistance 



1.00 

0.95 

0.90 

0.85 

0.80 

0.70 

0.60 

0.40 

2.5 

1.27 

1.91 

2.14 

2.28 

2.37 

2.52 

2.60 

2.71 

2.6 

1.30 

1.95 

2.20 

2.34 

2.44 

2.60 

2.67 

2.80 

2.7 

1.32 

1.99 

2.26 

2.41 

2.51 

2.68 

2.74 

2.98 

2.8 

1.35 

2.05 

2.32 

2.47 

2.57 

2.76 

2.82 

3.07 

2.9 

1.37 

2.10 

2.39 

2.54 

2.64 

2.83 

2.91 

3.15 

3.0 

1.40 

2.15 

2.45 

2.60 

2.72 

2.90 

3.00 

3.23 

3.1 

1.42 

2.20 

2.51 

2.66 

2.80 

2.97 

3.10 

3.31 

3.2 

1.45 

2.26 

2.57 

2.73 

2.87 

3.05 

3.20 

3.39 

3.3 

1.48 

2.31 

2.63 

2.80 

2.93 

3.12 

3.30 

3.47 

3.4 

1.51 

2.36 

2.69 

2.87 

3.00 

3.20 

3.39 

3.56 

3.5 

1.53 

2.42 

2.74 

2.94 

3.08 

3.27 

3.48 

3.65 

3.6 

1.57 

2.47 

2.80 

3.00 

3.15 

3.35 

3.56 

3.75 

3.7 

1.60 

2.52 

2.86 

3.07 

3.23 

3.43 

3.65 

3.85 

























400 


APPENDIX A 


TABLE 47 


Symbols for Wiring Diagrams 




5M 


Wires crossing 

Wires joined 

Incandescent lamps 


X Arc lamp 


—VWVWV” Resistance 



Fuse 


'{ 

n 


Knife switch- 
single pole 


Knife switch- 
double pole 



Knife switch—' 
triple pole 



Circuit breaker 


Transformer 

-ww— 



Shunt motor* 



* For generator use same symbol with G in center. 


Heights of Centre of Wall Outlets * 

(unless otherwise specified) 


Living-rooms.5' 6" 

Chambers.5' 0" 

Offices. 6' 0" 

Corridors.6' 3" 


Height of switches (unless otherwise specified).. 4' 0" 


* Recommended by the National Electrical Contractors’ Association, 
























APPENDIX A 


401 


STANDARD SYMBOLS FOR WIRING PLANS 
Adopted by the Nat’l Elect’l Contractors’ Ass’n and the Am. Inst, of Architects 


Ef 




If gas only 0 
If gas only Jj^(( 


X 

» 

® 

3 

coo 

S’ 

s 2 

s 3 

s 4 

s° 

s e 

a 


Ceiling outlet; electric only* 

Ceiling outlet; combinationf 
Bracket outlet; electric only* 

Bracket outlet; combinationf 
Wall or baseboard receptacle outlet* 

Floor outlet* 

Outlet for outdoor standard or pedestal; electric only* 

Outlet for outdoor standard or pedestal; combinationf 
Drop cord outlet 

One light outlet, for lamp receptacle 
Arc lamp outlet 

Special outlet, for lighting, heating and power current, as described 
Ceiling fan outlet 

Show as many symbols as there are 
switches, Or in case of a very large 
group of switches, indicate number of 
• switches by a Roman numeral, thus: S 1 
XII, meaning 12 single-pole switches. 
Describe type of switch in specifica¬ 
tions, that is, flush or surface, push¬ 
button or snap. 




M 

H 

a 

» 

Sto 

—4 

H§> 

—1 

-O 

HD 

m 

0 

I# 


S. P. switch outlet 
D. P. switch outlet 

3- way switch outlet 

4- way switch outlet 
Automatic door switch outlet 
Electrolier switch outlet 
Meter outlet 
Distribution panel 
Junction or pull box 

Motor outlet; numeral in centre indicates horsepower 

Motor control outlet 

Transformer 

— Main or feeder run, concealed under floor 

— Main or feeder run, concealed under floor above 
--Main or feeder run, exposed 

— Branch circuit run, concealed under floor 

— Branch circuit, concealed under floor above 

— Branch circuit run, exposed 

— Pole line 

Riser 

Telephone outlet; private service 
Telephone outlet; public service 
Bell outlet 

Buzzer outlet . 

Push-button outlet; numeral indicates number of pushes 

Annunciator; numeral indicates number of points 

Speaking tube 

Watchman clock outlet 

Watchman station outlet 

Master time clock outlet 

Secondary time clock outlet 

Special'Outlet; for signal systems, as described in specifications 
Battery outlet 


_Circuit for clock, telephone, bell or other service,: run under Actor, 

_Circuit for 1 dock, telephone, bell or other service,: run under floor 

above, concealed 

a xTnwiarol inrlir'ates number of 16 cp. incandescent lamps. 

+ numeral indicates number of 16 cp. incandescent lamps, lower nu¬ 

meral number of gas burners; e. g., * indicates 4 incandescent lamps and 2 gas 

bU f Kind of service wanted determined by symbol to which line connects. 
Copyright 1907 by the Nat’l Electrical Contractors’ Ass’n of the United States. 
















! 




' 








* 




■ 

















♦ 

. 


' ' 





























• • 

. 













; . t 





INDEX 


« 


Titles of articles are given in bold-face type when principal references; and 
in italics, when cross references. Numbers refer to pages. 


Absorption of light, 38, 39, 51 
Accessories for lighting, 51-63 
Adding loads having different power 
factors, 329 

Adjuster-socket system for street 
lighting, 112 

A. C. Systems, 198-202, 321-336 
arrangement of, 198-202 
calculation of load on, 324-331 
choice of, 204-207 
comparison, 202-203 
single-phase, 193, 324, 331-333 
three-phase, 198-200, 325-327, 334 
two-phase, 201, 327—329, 336 
voltages for, 205-207 

Aluminum for conductors, 248 
American wire gauge, 250 
Anchors, on vertical runs, 385 
Apparent power, 322 
Arc, electric, 17 
intensified, 24 

lamps, see Lamps, Arc, 17—36 
Armored Cable Systems, 226-229 
arrangement for fixture outlet, 73 
comparative cost, 244 
comparison with other systems, 242 
in finished buildings, 244 
in fireproof buildings, 210 
(see also Wiring, Interior) 

Armored cord, 258 
Attachment plugs, 284 
Auto-starters, 135, 166, 169 

B. & S. gauge, 250 

Backing, for metal moulding, 229 
for wood moulding, 232 


Balanced system, 
three-phase, 198, 199 
three-wire, 196 
two-phase, 202 

Ballast, for Cooper Hewitt lamps, 32 
Belting, horsepower of, 380 
Belts, leather, 186 
rubber, 187 
Bending conduit, 211 
Bends, conduit, 211, 382 

anchoring flexible conduit, 225 
number allowed in run, 218 
offset, 218 

Bi-metallic wire, 248 
Blowers and fans, motor requirements 
of, 196 

horsepower required for, 182 
Bowls, selecting, 102 

for semi-indirect systems, 70 
translucent, for semi-indirect sys¬ 
tems, 61 

translucent, applications, 62 
(see also Reflectors, 51-62) 

Braids, for wires and cables, 254 
Braking, dynamic, 164 
Branch circuits, 
arranging, 295 

calculating voltage loss on, 320 
circuit breakers on, 315 
conduit for, 210 
control of, 286, 299 
definition, 293 

for factories, methods of installing, 
235 

for an office building, 341 
fusing a.c., 329 


403 





404 


INDEX 


Branch circuits, continued, 
fusing d.c., 314 
lighting, 302, 303 
for arc lamps, 315 
calculating load on, 309 
length of, 392 
motor, 303, 309, 315 
number allowed in one conduit, 219 
size of a.c., 329 
size of d.c., 314 

sizes when in moulding, 230 % 

Brown and Sharpe gauge (B. & S. 
gauge), 250 

Busbars, for panel boards, 286, 287 
Bushing ends of conduit, 215 
Bushings, dimensions of, 383 
flexible conduit, 225 
rigid conduit, 216 
use at fixture outlets, 74-76 
BX cable, BXL cable, see Armored 
Cable, 226-229 

Cabinets, for panel boards, 287, 390 
Cable, cables, 248-260 

armored, see Armored Cable, 226-229 
concentric laid, 252 
connectors for, 259 
definition, 252 
dimensions of bare, 387 
extra-flexible, 253 
rope laid, 252 
sizes used, 252 
soldering, 259 
splicing, 259 
stranding of, 253 
(see also Conductors, Wires) 
Cambric, varnished, 256 
Candle, international, 39 
Candlepower, 39-41 

at different distances, 41 
comparing different lamps, 40 
definition of, 39 
effect of reflectors upon, 52 
of lamps, see under name of lamp 
mean horizontal, 40 
mean lower hemispherical, 41 
mean spherical, 41 
Canopy for fixtures, 66, 72 
Capping for metal moulding, 229 
for wood moulding, 231 
Carbon, carbons. 

cored, for arc lamps, 22 


Carbon, carbons, continued, 
for flame arc lamps, 28 
lamps, 8 

Cement mills, motor requirements for, 
184 

Central station supply, voltages used 
for, 205 ^ 

Centrifugal fans, motor requirements 
of, 196 

Centrifugal pumps, motor require¬ 
ments of, 180 
Chain drives, 186, 189 
Chart, for calculating size of circuits, 
318 

Circuit breakers, 268-273 
air-break type, 268-271 
applications, 270 
compared w r ith fuses, 279 
compared with oil circuit break¬ 
ers, 273, 291 
construction, 268 
operation on momentary over¬ 
loads, 270 
overload type, 269 
sizes, 270 

use in dusty places, 271 
oil, 271-273 

action on overloads, 272 
applications, 273 

comparison with air-break type, 
273, 291 

construction, 271 
not used on d.c., 271 
setting, to protect a circuit, 271, 272, 
315 

“ time-limit ” feature, 272 
use to protect motors, 185 
Circuits, 309-336 

a.c., calculating load on, 321-329 
calculating voltage drop, 331-336 
grouping wires, 219 
single-phase, 193 
size of, 329-331 
three-phase, 198-200 
two-phase, 201-202 
arrangement of, 293-308 
branch lighting, length of, 392 
different systems not allowed in the 
same conduit, 219 
d.c., calculating load on, 309-315 
calculating voltage drop, 316-320 
size of, 315-316 



INDEX 


405 


Circuits, continued, 

d.c., three-wire, 193-197 
three-wire convertible, 197 
two-wire, 193 
grounding of, 307 
largest size in conduit, 306 
number allowed in one conduit, 219 
parts of a, 293 

single-phase, see Circuits, a.c., 193 
size of, 309 
special lighting, 300 
three-phase, see Circuits, a.c., 198- 
200 

three-wire, see Circuits, d.c., 193- 

197 

two-phase, see Circuits, a.c., 201-202 
two-wire, see Circuits, d.c., 193 
Circular mils, 250, 251 
Cleats, dimensions of porcelain, 386 
for open wiring, 236 
Code of lighting, 84 
Code, National Electrical, 208 
Color, colors 
caps, 7 

classification of walls and ceiling, 
368 

of an object’, 38 
primary, 37 

Commutating poles, 131, 149 
Compensating winding, used with 
interpole machines, 132 
Compensators, or Auto-starters, 135, 
166, 169 

Compressors, air-, motor requirements 
* • for, 182 
Concentric wire, 228 
Concrete ceilings, .fixture outlets on, 
74-75 

Condensation in conduit systems, 246 
Conductors, 248-260 
aluminum, 248-249 
carrying capacity of a.c., 258, 322 
copper for, 249 

effect of voltage on size of, 192 
grouping of a.c., 322 
insulated, 253-258 
materials for, 248 
maximum sizes of a.c., 323 
multiple, 257 
solid, 252 
stranded, 252-253 
(see also, Cables, Wires) 


Conduit, 209-226 

flexible, 223-226 
applications, 223 
bushings, 225 
condensation in, 246 
construction, 223 
in brick walls, 224 
in concrete walls, 224 
in finished buildings, 244 
in fireproof buildings, 210 
fittings for, 225 
installation of, 225 
outlets for, 224 
sizes manufactured, 224 
sizes for given wire, 225, 384 
systems, comparison with other 
systems, 242 
wire used, 225 

(see also Wiring, Interior, 208- 
247) 

rigid, 209-221 

anchoring vertical runs, 222 
applications, 210 
bending, 211 
bends, 211, 382 
bushings for, 216, 383 
bushing ends of, 215 
circuits in conduit, 219 
cleaning out before pulling wires, 
223 

comparison with other systems, 
241 

condensation in, 246 
construction of, 210 
couplings for, 217 
description, 209 
dimensions of, 382 
elbows, 211, 382 
exposed, 216 
for wet places, 245 
galvanized vs. enameled, 223 
grounding, 223 
in finished buildings, 244 
installing, 219 
installing wire in, 222 
lined, 211 

locknuts for, 216, 383 
maximum size of circuit, 306 
pipe straps for, 221 
pull boxes for, 219, 385 
relative cost of, 244 
sizes of, 210 




406 


INDEX 


Conduit, continued, 

sizes required for wires, 218, 384 
supports for, 221 
supports for fixtures, 74-76 
wire used, 217 

(see also, Wiring, Interior, 208- 
247) 

Condulets, 216 
Connectors, Dossert, 259 
fixture wire, 259 
for flexible conduit, 225 
Constant-current system, 111, 190 
Constant-potential system, 112, 191 
Control equipment for motors, 185 
Controllers, drum type, 161 
motor, d.c., 158 
speed, 159 

(see also Rheostats, Speed Regulators, 
Starters) 

Convertible system, three-wire, 197 
Cooper Hewitt lamps, see Lamps, 
Mercury vapor, 30-35, 371 
Copper, 249 

Copper-clad steel wire, 248 
Cord, flexible, 257-258 
Coupling of motors, direct, 186 
Couplings, for flexible conduit, 225 
for rigid conduit, 217 
Cove lighting, 62 

Cranes, motor requirements for, 183 
Crow-foot for fixtures, 74 
Current, currents, 

carrying capacity of wires, 389 
for d.c. motors, 375 
for three-phase motors, 376 
for two-phase motors, 377 
rating of a.c. conductors, 322 
Cutout cabinets, 286 
Cutouts, automatic, for arc lamps, 18 
enclosed fuse, 277 
film, 113 
lamp, 113 

Daylight, artificial reproduction of, 39 
Demand factor, for motor loads, 312, 
391 

Diagrams, symbols for, 400 
Differential motor, 130 
Diffusion, 37, 38 
Direct drives, 186 
D. C. Systems, 193-198, 309-320 
arrangement of, 193 198 


D. C. Systems, continued, 
calculating load on, 309-314 
choice of, 204-207 
comparison, 202-203 
three-wire, 193-198, 309-320 
two-wire, 193, 309-320 
voltages for, 204-208 
Disk fans, motor requirements for, 196 
Distribution, centre, 293 
curves, 45 
methods of, 190 
systems, comparison of, 202 
for street lighting, 111 
(see also, A. C. Systems, 198-202, 
321-336. D.C. Systems, 193- 
198, 309-320) 

Dossert connectors, 259 
Drives, belt, 186, 187, 188 
chain, 186, 189 
direct, 186 
for tanneries, 184 
gear, 186, 188 

group, 172, 174, 177, 185, 346 
individual, 172, 173, 177, 346 
rope, 186 

Drop factor, for a.c. circuits, 333, 
398 

Drop, see Voltage Drop 
Dust, effect of, on reflectors, 59, 85 
Duplex wire (Copper-clad), 248 
Dynamic braking, 164 

Edison system, see D.C. Systems, 193- 
198, 309-320 

Efficiency, of lighting installations, 
50, 57, 61, 62 
of motors, 151, 176 
utilization, of incandescent lamps, 
92, 367 

Electric drive, advantages of, 124 
methods, 172 
see also Drives. 

Electrodes, for open arc lamps, 17, 19 
for enclosed arc lamps, 22 
for flame-arc lamps, 24 
for metallic-electrode arc lamps, 28 
Elbows, conduit, 211, 382 
Electroliers, control of lamps on, 301 
Elevators, motor requirements for, 
182 

. Eye, adaptability to different light 
intensities, 47 



INDEX 


407 


Fans, motor requirements for, 196 
Feeders, 293-298, 313-315 
a.c. determining size of, 331 
fusing of, 331 
run in parallel, 323 
voltage loss, 333 
arrangement of, 296-298, 306 
definition, 293 
d.c. determining size of, 315 
fusing of, 315 
voltage loss, 320 

for factories, methods of installing, 
235 

for hall lamps, etc., 198 
for an office building, 297, 337 
fuses for, 292 
lighting, 311, 339 
power, 313, 340 

separate from lighting, 298 
protecting, 290 
riser diagram, 306 
size of, 306 

Film cutout, for series systems, 
113 

Fish-wire, for pulling in wires, 222 
Fittings, list of electrical, 208 
Fixture studs, 76 
Fixtures, Lighting, 64-77 
canopies, 72 

control of lamps on, 301 
for direct systems, 64-70 
for indirect systems, 71 
for street lighting, 112 
hanging height for indirect systems, 
101 

insulating joints, 71 
mounting height for direct systems, 
100 

outlet boxes for, 214 
semi-indirect, 68-70 
spacing of, 96 
supports for, 73-77 
wall brackets, 70 
wire for, 255 

for large gas-filled units, 256 
wiring, 66, 69 

for large gas-filled lamps, 247 
Flashers, sign, 119, 121 
Flexible conduit, see Conduit, flexible, 
209-226 

Flexible tubing, 224, 235 
use at fixture outlets, 73, 74 


Flywheel, use with compound motor, 
130 

Foot-candles, 42 

Four-wire, three-phase system, see 
A.C. Systems 

Frequency, effect on motor speeds, 154 
effect on self induction, 321 
effect on voltage loss, 321 
for industrial service, 207 
standard, for motors, 147 
Frosted lamps, 6, 69 
use in signs, 116 
Fuses, 273-279 

action on overloads, 276 
applications, 278 
cartridge, 275 

compared with circuit-breakers, 279 

cutouts for, 277 

enclosed, 274 

for a.c. motors, 171 

for branch circuits, 295 

for induction motors, 167 

for motors, 391 

for panel boards, 286 

link, 273 

open, 273 

plug, 274 

rating of, 277 

refilled, enclosed, 277 

renewable, enclosed, 277 

in rosettes, 284 

“ running,” for a.c. motors, 330 
sizes of enclosed, 276 
“ starting ” for a.c. motors, 330 
“ time limit ” feature of, 270, 276 
Fuse wire, 273 

Fusing, a.c. branch circuits, 329 

Galvanizing steel, 211, 219 
Gas check, for arc lamps, 20, 21 
Gauges, wire, 250 
Gear drives, 186, 188 
Gem lamps, 9, 353 
Generators, arc light, 111 
grounding frames of, 308 
standard voltages for, 204 
Getter, 11 
Glare, 49 
Globes, 

arc lamps, 63, 355, 356 
diffusing, 60 
purpose of, 53 





408 


INDEX 


Globes, continued, 
selecting, 102-103 
vapor-tight, 247 

Greenfield conduit, see Conduit, Flex¬ 
ible, 224 

Ground connections, 223 
Grounding, of circuits, 307 
metal moulding, 231 
rigid conduit systems, 223 
Group drives, 172, 174, 177, 185, 34G 
Gutters, for panel boards, 287 

Hall lamps, control of, 298, 300 
Hangers, for fixtures, 74-76 
Hanging height, for lamps used for 
indirect systems, 101 
Head, on a pump, 180 
Heating, of incandescent lamps, 15 
Hickey, for bending conduit, 212 
fixture, 73-74 

Hoists, motor requirements of, 183 
Holders, for reflectors and shades, 57 
Horsepower, of belting, 187, 380 
for blowers and fans, 182 
converting to kilo-watts, 148 
for group drives, 185 
for hoists, 183 
for machine tools, 178 
for pumps, 181 

Hotel, lighting system for a, 348 

Illumination, 37-50, 78-110 
of benches, 100 
calculating, 78-110 
direct, see Lighting, 46 
examples of calculation of, 104-110 
effect of change of height, 88 
indirect, see Lighting, 46 
intensity, 47, 84, 92 

for commercial lighting, 359 
effect of dark material, 85 
for industrial lighting, 84, 362 
for street lighting, 114, 373 
methods of, 79-83 
mounting height of lamps, 100 
oblique rays, 45 

power required for, 92-95, 368-370 
principles of, 37-50 
requirements for artificial, 46 
semi-indirect, see Lighting, 46 
spacing of units for uniform, 97 
of signs, intensity, 123 


Illumination, continued, 
systems, 46, 83 
uniform, 86-104 

power required, 92-94 
securing, 86, 88 
uniformity of, 48 
variation with distance, 41, 44 
(see also Lighting) 

Incandescent lamps, see Lamps, Incan¬ 
descent, 4-16 

Individual drives, 172, 173, 177, 346 
Inspection of wiring, 208 
Insulating joints for fixtures, 73, 74 
Insulator rack, 236 

Insulators, for knob and tube wiring, 
234 

for open wiring, 236 
Intensified arc, 24, 355 
Interpoles, 131, 149 
Inverse square law, 45, 101 
Iron, for conductors, 248 
Isolated plants, voltages used for, 206 

Jointing wires, 259 

Joints, wire, for wet places, 245 

Junction point, on feeder system, 293 

Kick block, for moulding, 233 
Kilowatts, converting to horsepower, 
148 

Knob and tube systems, 234-235 
comparison with other systems, 242 
in finished buildings, 244 
outlets, 73, 74, 234 
relative cost, 244 

Knobs, for knob and tube wiring, 234 
for open wiring, 236 
Knockouts, 214 
Knot, for pendant cord, 281 

Labels, for approved fittings, 208, 255 
Lamps 

Arc, 17-36 

branch circuits for, 315 
construction of, 17 
cutouts, 18 
enclosed, 20-24, 355 
flame-arcs, 24-28, 356, 370 
for general lighting, 80 
intensified arc, 24, 335 
light efficiency, 2 
luminous, 28 





INDEX 


409 


Lamps, continued, 
magnetite, 28 

metallic-electrode arcs, 28-30, 357 
open type, 19 
operation on 25 cycles, 27 
power required for illumination 
by, 95 

rating, 18, 41 
reflectors and globes, 03 
on series systems, 111 
for street lighting, 112 
types, 3 

Cooper Hewitt, see Lamps, Mercury- 
vapor, 30-35 
Incandescent, 4-16 
arrangement of, 99 
bases, 4, 6 

for lighting bill-boards, 123 
blue-glass, 7 
calculating load of, 309 
carbon-filament, 8 
coil filament, 10 
color of light, 7, 14 
colored, 7 
comparing, 0 
concentrated filament, 13 
control of, 299, 300 
control from several points, 265 
distribution of light, 8 
effect of alternating current, 7 
effect of excessive temperature of 
filament, 5 

effect of excessive voltage, 5 
frosted lamps, 6 
Gem, 9, 353 
heating effect, 15, 58 
life, 4 

life of frosted, 7 
Mazda B, 10-12, 353 
Mazda C, 12-16, 354 

see also Lamps, Incandescent, 
Tungsten 

metallized-nlament, 9, 353 
mounting height, 100 
power consumption, 5 
power required for illumination 
with, 92-95, 368—369 
rating, 5, 40 

sizes of, for various heights of 
mounting, 370 
spacing, 96 

for street lighting, 111 i 


Lamps, continued, 
tantalum, 9 
tungsten, 1016 
applications, 84 
current rush at starting, 16 
data on, 353-354 
gas-filled, 12-15 
light efficiency, 1 
overshooting, 15 
power required for illumination 
by, 92-95, 368-369 
for signs, 116 
for street lighting, 112 
utilization efficiencies for, 367 
vacuum type, 1012 
voltage variation, effect of, 15 
types, 2, 3 

wire-drawn filament, 10 
Mercury-vapor, 3035, 358 
applications, 34, 84 
construction, 30 
current rush at starting, 35 
data on, 358 

distribution and color of light, 35 
efficiency and tube life, 34 
standard sizes, 34 
Lights, see Lamps 
Light, absorption of, 38, 39, 51 
color, 37, 49 

enclosed arc, 24 
mercury vapor lamps, 35 
open arc, 20 

composition of white, 37 
diffusion of, 37, 38 
efficiency, 1 
flickering of, 48 
flux, definition, 41 
production, 1, 37 
quality of, 39 
reflection of, 38 
refraction of, 37, 38 
Light transformers, for mercury-vapor 
arcs, 35 

Light, ultra-violet, 35, 39 
units of, 39-45 
candlepower, 41 
foot-candles, 42 
lumens, 41 

Light-units, 64-77, 85-102 
arrangement of, 99 
for direct lighting, 64-70 
for indirect lighting, 71 




410 


INDEX 


Light-units, continued, 
location of, 96-102 
for semi-indirect lighting, 70 
size of, 96, 370 
spacing of, 96, 372, 373 
for streets, 112, 115 
for yard lighting, 115 
Lighting, 

accessories, 51—63 
circuits, a.c., 332 
d.c., 315 
protecting, 279 
code of, 84 
cove, 62 

direct system, 83 

calculation of, 86-110 
fixtures for, 64-70 
reflectors for, 53-61 
factory, 341 

fixtures, see Fixtures, 64-77 
flood, 122 

frequencies, suitable for, 48 
general, definition, 80 

power required for, 92-94 
group, 81 
tor a hotel, 348 
indirect system, 83 
calculation of, 86-110 
fixtures for, 71 
reflectors for, 62 
installation, appearance, 50 
efficiency, 50 

intensities for commercial, 359 
for industrial, 362 
local, 79 

effect on eye, 48 
power required for, 95 
localized-general, 81 
machine shop, 108, 342 
office, 104 
outdoor, 111-123 
power required for, 368, 371 
railroad repair shop, 344 
residence, 348 

semi-indirect system, 46, 83 
calculation of, 86-110 
fixtures for, 70 
reflectors for, 61 
service, voltages used for, 205 
sewing machines, 82 
standards for streets, 114 
store, 106 


Lighting, continued, 
street, 111—115 

systems, branch circuits for, 294 
efficiency, 61, 62 
operating costs, 84 
tennis courts, 116 
yard, 115 

(see also Illumination ) 

Lines, distribution, 205 
Loads, calculation of a.c., 324-329 
calculation of d.c., 309-315 
Load centre, locating, 318 

determining requirements, 177 
Locknuts, conduit, 216, 383 
use at fixture outlets, 74-76 
Loom, circular, 235 
Low-voltage release for motor starters, 
157, 159, 166 
Lumens, definition, 41 
Luminous-arc lamp, 28 

Machine, shops, wiring system for, 346 
tools, power requirements for, 147, 
177-179 

Machines, Operating Requirements of, 

177-185 

blowers and fans, 182 
cement mills, 184 
cranes, 183 
dusty places, 150 
elevators, 183 
group drives, 185 
hoists, 183 

industrial purposes, 145 
machine tools, 147, 177, 179 
pumps, 181 
steel mills, 184 
tanneries, 184 
textile mills, 184 
wood-working machinery, 179 
Magnetite arc, 28 
Mains, 

arrangement of, 296, 306 
definition, 293 
fusing, 297, 315, 331 
lighting, 294 
load on, 311, 313 
for an office building, 340 
size of a.c., 331 
size of d.c., 315 
voltage loss on, 320, 333 
Mazda B lamps, 10-12, 353 




INDEX 


411 


Mazda C lamps, 12-16, 354 

Mazda, daylight, 15 

Metallic flame arc, see Lamps , Arc, 28 

Meters, for switchboards, 292 

Mils,- circular, 250, 251 

Moore-tube lamps, 35 

Motor, Motors, 

Alternating Current, 133-145 
for adjustable-speed service, 136 
applications of, 135, 136, 144 
BK type, single-phase, 142 
branch circuits for, 329 
brush-shifting type, 136 
calculating load of, 322, 327, 328 
commutator type, 136, 140 
compared with d.c., 143 
connecting to circuit, 327 
currents for, 152, 327, 328, 376, 
377 

frequency for, 147, 154 
fuses for, 156, 167, 171, 329, 330, 
391 

induction, polyphase, 133-137, 
175 

induction, single-phase, 137-138 
for industrial purposes, 206 
multi-speed induction, 136 
overload capacity, 149, 374 
power factor of, 134, 153, 378 
“ pull out ” point, 149 
RI type, 141 
repulsion, 140 
reversing rotation of, 135 
shaded pole, 138 
single-phase, 137-142, 324 
slip-ring, induction, 133, 135 
speed-regulation of, 134, 136, 140 
speeds of, 147, 175, 378 
split-phase, 138 

squirrel-cage induction, 133, 134 
starting currents of, 152, 330 
starting methods, 155 
starting torque, 152 
synchronous, 139, 144, 149, 152, 
153 

three-phase, 133-137 
torque, 144, 146, 154 
two-phase, 133-137 
voltages for, 146, 205 
wire size for, 376, 377 
Applications, 172-189 
blowers and fans, 182 


Motor, Motors, continued, 
cement mills, 184 
cranes, 183 
dusty places, 150 
elevators, 183 
group drives, 185 
hoists, 183 

industrial purposes, 145 
machine tools, 147, 177, 178 
method of connecting to load, 186 
office building, 340 
pumps, 181 
steel mills, 184 
tanneries, 184 
textile mills, 184 
wood-working machinery, 179 
auxiliary apparatus required, 156 
branch circuits for, 309, 315 
choosing type of, 174 
circuit breakers for, 270 
classes and types, 125 
classification for performance, 174 
classification for speed regulation, 
125 

comparison of a.c. and d.c., 145, 206 

control devices required, 300 

for damp places, 150 

demand factors for, 312, 391 

differential, 130 

Direct Current, 126-133 

for adjustable speed service, 146 
applications of, 127, 129-132 
branch circuits, 315 
calculating load of, 309 
commutating pole, 131, 149 
comparison with a.c., 143 
compound, 129, 130 
currents for, 375 
differential, 130 
dynamic braking with, 164 
fuses for, 156, 315, 391 
interpole, 131, 149 
overload capacity, 149, 374 
reversing rotation, 127 
running performance, 133 
series, 126 
series-shunt, 131 
shunt, 127-129 
speeds of, 175, 378 
speed regulation of, 126, 128, 130, 
132 

starting current, 151, 152, 313 





412 


INDEX 


Motor, Motors, continued, 
starting methods, 155 
starting torque of, 152 
starting without rheostat, 155 
torque, 152 
voltages for, 146, 205 
wire sizes for, 375 
dust-proof bearings, 175 
efficiency, 176 
enclosed, 150 
grounding frames of, 308 
input, 148 

load and motor rating, 176 
open, 150 

vs. enclosed, 175 
ordering, 189 
output, 148 
performance, 151 
protecting against overload, 279 
pulley sizes for, 379 
rating, 148 
semi-enclosed, 150 
speed, effect upon cost and weight, 
175 

ratings, 378 

ratio to speed of load, 176 
systems used with, 125 
temperature ratings of, 149, 374 
vertical, 143 

voltage variations, effect of, 153 
Moulding, metal, 229-232 
applications, 229 
construction, 229 
fittings for, 230 
installation of, 231 
wood, 232-233 

for exposed work, 245 
Mounting height of lamps, 100 
of street lamps, 115 
Multiple system, 125, 190 

National Electrical Code, 208 
Neon-tube lamp, 36 • 

Neutral, three-phase, 199 
three-wire, 196, 315, 316 
for three-wire convertible system, 
197 

Nitrogen lamps, see Lamps, Incan¬ 
descent, 12-16 

Office building, wiring for, 337-341 
Open wiring, see $Viring, Open, 235- 
241 


Operating requirements for machines, 
see Machines, 177—185 
Outlet, outlets, 

for armored cable, 227 
boxes, 213-216 
for fixtures, 73-76 
setting, 220 

for flexible conduit, 224 
for metal moulding, 230 
for an office building, 339 
plates, 214 
wall, heights of, 400 
for wood moulding, 233 
Overload, capacity of motors, 148, 374 
Overload release, for motor starters, 
158, 159, 167 

Overshooting, of tungsten lamps, 15 

Panel boards, 285-289 
cabinets, 287, 289, 390 
frames, 280 
fuses, 286 
gutters, 287 
lighting, 286 
load on lighting, 309 
on power, 312 
location of, 304 
methods of feeding, 296-298 
number on one feeder, 297 
number of circuits on, 289 
for an office building, 337 
power, 289 
size of, 288, 304 
spare circuits on, 304 
switches for, 262, 286 
Panel box, see also Panel Board, 293 
Paper insulation, for wires, 256 
Para rubber, 255 
Pendant or drop, 64 
Phase-wound motor (slip ring), 135 
Pipe, see Conduit, Rigid, 209 
Pipe straps, 221 
Plunger pumps, 180 
Polyphase systems, 198-207, 325-336 
(see also A.C. Systems) 

Power, apparent, 152, 322 
to drive machines, 177-183 
factor, of apparatus, 394 
definition, 322 
effect on voltage drop, 321 
of induction motors, 378 
of motors, 152 



INDEX 


413 


Power, apparent, continued, 
real, definition, 322 
supply, branch circuit arrangement, 
295 

factory, 341 

industrial plants, 207 

machine shop, 346 

methods, 190 

systems, 125, 190 

(see also A.C. Systems D.C. 

Systems) 

voltages used, 205 
(see also Horsepower) 

Projectors, flood lighting, 123 
Propeller fans, 181 
Pull boxps, for conduit, 219, 385 
Pulley sizes for motors, 188, 379 
Pumps, horsepower to drive, 181 
motor requirements for, 180 

Quarter-phase system, see A.C. Sys¬ 
tems, Two-phase, 201 
Quartz-tube lamp, see Lamps, Mer¬ 
cury-vapor, 33 

Rating of electrical machinery, 148- 
150 

Reactance, ratio to resistance, 395 
Reactive factors, 329, 394 
Real power, definition, 322 
Receptacles, 282-284 

number allowed on branch circuit 
302 

Reciprocating pumps, 180 
Rectifiers, 111 

Reflection, efficiency, for walls and 
ceilings, 61 
of light, 38 
Reflectors, 51-62 
angle type, 54, 122 
applications, 59 
arc lamp, 24, 28, 63 
change of candlepower of lamp, 42 
construction, 55 
for direct systems, 53 
distribution of light, 43, 52, 86 
dust on, 59, 85 
efficiency, 57, 92 
examples of, 56 
glass, 52, 55, 57 
holders for, 57 
indirect systems, 62 


Reflectors, continued, 

modify inverse square law, 45, 110 
light distribution, 43, 52, 86 
for mercury-vapor lamps, 35 
purpose of, 51 

“ red,” for mercury-vapor lamps, 35 
selecting, 102-103 
semi-indirect systems, 61 
size of, 57 

spacing and mounting height, 92 
steel, 54, 55 
surfaces for, 53 
(see also Bowls) 

Refraction, of light, 37 
Refractor, for arc lamps, 29 
Regulation, speed, of motors, see 
under Motors 
Regulators, speed, 159 
Relays, 272 

Residence, lighting system for a, 348 
Resistance factors, 329, 394 
Resistance, formula for calculating 
resistance of a wire, 251 
Resistance, see Starters, Motor 
Rheostats, used with series motors, 127 
used with shunt motors, 128 
see also Controllers, Speed Regulators, 
Starters. 

Rhodamine enamel, 35 
Rigid conduit, see Conduit,. Rigid, 
209-223 

Riser diagram, 306, 310 
for an office building, 339 
for power system, 313 
Risers, anchoring vertical, 222 
Rope drives, 186 
Rosettes, 283 
fused, 284, 294 

Rotor of induction motor, 133 
Rubber insulation, 253 

compared with other types, 256 
effect of high temperature on, 255 
tests of, 255 

Self-induction, on a.c. circuits, 321 
Series system, 190-191 
not used for motors, 125 
for street lighting, 111 
Service, services, 
installation of, 294 
location of, 304 
main, 293 







414 


INDEX 


Service, services, continued, 
for office building, 337 
for residence, 306 
voltages used for, 205 
Shade, shades, 53 
holders, 58, 102 
(see also Reflectors) 

Shadows, eliminating, 49 
Shafting drives, losses in, 124 
Sheradizing steel, 214 
Shifter, for mercury-vapor lamps, 31, 
32 

Shock, absorbers for fixtures, 66 
electric, danger from, 146 
Shunt motor, see Motors, d.c., 127-129 
Signs, electric, 116-123 

advantages of tungsten lamps for, 16 
connection of a.c., 118 
connection of d.c., 117 
connection of transformers for, 122 
flashers, 119, 121 

Single-phase systems, see A.C. Sys¬ 
tems, 193, 324, 333 
Skin effect, 321, 393 
Slip-ring motors, see Motors, a.c., 135 
Slow-burning insulation, 256 
weatherproof insulation, 256 
Snake, for pulling wires, 222 
Sockets, 280-283 

number allowed on a branch circuit, 
302 

Soldering, flux for, 260 
wires, 259 

Spacing of direct lighting units, 372 
of indirect and semi-indirect units, 
373 

light-units, 96 
of street lamps, 115 
three-phase circuits, 335 
of wires, effect on self induction, 321 
Speed, of motor and driven machine, 
limits, 176 

ratings, of motors, 378 
regulators, 156, 159 

(see also Controllers, Rheostats, 
Starters) 

Splices, taping wire, 260 
for wet places, 245 
Splicing wires, 259 

Squirrel-cage motor, see Motors, a.c., 
134 

Square mils, 250 


Star-delta starting, 170 
Starters, 

A.C. Motor, 134-136, 166-171 
auto-starters, 135, 166, 168 
low-voltage release, 166 
overload release, 167 
rating of, 156 
resistance type, 136, 168 
selecting, 185 
size required, 156 
star-delta, 170 
switches, 171 

(see also Controllers, Rheostats, 
Speed Regulators) 

D.C. Motors, 126-132, 156-166 
automatic, 162 
compound, 160 
dash-pot type, 162 
drum controllers, 156, 158 
face-plate type, 157 
low-voltage release, 157, 159 
multiple-switch, 158 
overload release, 158, 159 
rating of, 156 
selecting, 185 
size required, 156 
(see also Controllers, Rheostats, 
Speed Regulators) 

Stator, of induction motor, 133 
Steel mills, motor requirements for, 
184 

Stranded conductors, 252 
Street lighting, 111-115 
arc lamps for, 20, 30 
intensities for, 47, 373 
Sub-feeders, definition, 293 
Switchboards, 289-292 
clearance around, 306 
location of, 304 
purpose of, 285 
Switches, 261-268 
bases, 266 

boxes, used with flush, 265 
for controlling branch lighting cir¬ 
cuits, 299 
door, 267 

double-pole, where required, 299 
knife, 261-264 
master, 301 

momentary contact, 267 
for motors, 300 

oil, see Circuit breakers, Oil, 272 




INDEX 


415 


Switches, continued, 

outlet boxes for, 214-215 
for panel boards, 286 
pendant, 267 
plates, 266-267 
push-button, 266-267 
push-button, special type, 301 
remote control, 267 
service, 263 

single-pole, where allowed, 299 
snap, 264, 266 
special, 264, 266 
three-way, 265 

Synchronous motor, see Motors, a.c. 

139 

Tanneries, motor requirements for, 184 
Tantalum, lamp, 9 
Tap, definition, 293 
Tape, for splices, 254, 260 
Temperature rise of motors, 374 
Terra-cotta ceilings, outlets on, 75 
Textile mills, motor requirements for, 
184 

Thermo-flashers, for signs, 119 
Three-phase system, see A.C. Systems, 
198-200, 325, 327, 334 
Three-wire system, see D.C. Systems, 
193-198, 309-320 

“ Time limit ” features, of circuit 
breakers, 272 
of fuses, 270 

Transformers, auto-, for motors, see 
Starters, a.c., 166 

constant-current for arc lighting, 

NK| 111 
for signs, 119 

Tubes, for knob and tube wiring, 234 
used in open wiring, 237 
Tungsten lamps, see Lamps, Incan¬ 
descent, 10-16 

Two-phase system, see A.C. Systems, 
201, 327-329 

Two-wire system, see D.C. Systems, 
193, 309-320 

Unbalanced load, on three-phase sys¬ 
tem, 199 

on three-wire system, 196 
Underwriters’ knot, 281 
Units, see Light, Units of, 39-45, and 
Light-units 

Utilization efficiency, 92, 367 


Vacuum getter, 11 

Vacuum-tube lamps, see Lamps, Arc, 
35 

Vacuum-type lamps, see Lamps, In¬ 
candescent, 10 

Varnished cambric, insulation, 256 
Ventilation of motors, 150 
Voltage 

drop, allowable, 316, 392 
a.c., calculation of, 331-336 
with different power factors, 333 
d.c. calculating, 316-317 
effect on cost of wiring, 192 
standard for motors and generators, 
146, 204 

Volt-amperes, definition, 322 
Vulcanizing, rubber insulation, 253 

Wall brackets, 70 
Watts, 322 

Weatherproof insulation, 255 
Weatherproof insulation, slow burn¬ 
ing, 256 
Wires, 248-260 
aluminum, 248 
anchoring open, 240 
anchoring vertical runs, 221 
for armored cable, 226 
bi-metallic, 248 
braids for, 254 
concentric, 228 
connectors for, 259 
copper, 249 
copper-clad, 248 

current-carrying capacity of, 389 

dimensions of insulated, 388 

duplex, 252, 257 

fixture, 255, 256 

for flexible conduit, 225 

fuse, 273 

gauges, 250-251 

grouping a.c. in same conduit, 219 
installing in rigid conduit, 222 
insulated, dimensions of, 388 
insulator racks for, 236 
for metal moulding, 230 
for open work, 238 
protection at crossing points, 237 
protection, for open wiring, 236 
resistance calculation, 251 
rubber insulated, 253-255 

compared with slow burning, 239 




416 


INDEX 


Wires, continued, 

for iron conduit, 217 
for open wiring, 238 
in wet places, 245 
size of cleats or knobs for, 23G 
size of, for d.c. motors, 375 
size of, for three-phase motors, 376 
size for two-phase motors, 377 
sizes of conduit for, 384 
slow-burning, 217, 256 

compared with rubber, 239 
soldering, 259 
solid, 252 

spacing in open work, 240 
splicing, 259 

stranded, sizes used, 217 
twin, 252, 257 
“ Underwriters,” 256 
weatherproof, 255 
for wood moulding, 233 
(see also Cables, Conductors ) 

Wiring, interior, 208-247 
anchoring vertical runs, 385 
armored cable, 226—229 
for battery rooms, 246 
“ breaking around ” beams, 240 
chart, for d.c. circuits, 318 
cleat, see Wiring, Open, 235-241 
concealed, rosettes for, 283 
conduit, 209-226 

when exposed to corrosive vapors, 
256 

exposed, see Wiring, Open, 235-241 
finished buildings, methods, 244 
flexible conduit, 223-225 


Wiring, interior, continued, 
for high temperatures, 247 
when exposed to inflammable gases, 
247 

installation methods, 208-213 
knob and tube, 233-235 
metal moulding, see Moulding, 229, 
230 

Open, 235-241 
applications, 235 
cleats and insulators for, 236 
compared with other systems, 243 
for finished buildings, 245 
installation of, 240 
maximum size of circuit, 307 
protection of, 236, 238 
relative cost, 244 
rosettes for, 283 
plans, symbols for, 401 
rigid conduit, see Conduit, 209—221 
for severe conditions, 245 
systems, 190—207 
choice of, 204 
comparison of, 241 
examples of, 337—351 
with mains and feeders, 294 
relative cost, 243 
for tungsten lamps, 15 
for wet places, 245 

Wood moulding, see Moulding, Wood, 
232 

Wood-working machinery, motor re¬ 
quirements for, 179 

Working plane, 85 


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